E ec2 transit gateway update params

[philnichol]

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Closes #15380

Release note for CHANGELOG:

ec2_transit_gateway options no longer ForceNew unless required to

Output from acceptance testing:

$ TF_ACC=1 go test ./aws -v -count 1 -parallel 4 -run=TestAccAWSEc2TransitGateway_ -timeout 60m
=== RUN   TestAccAWSEc2TransitGateway_basic
=== PAUSE TestAccAWSEc2TransitGateway_basic
=== RUN   TestAccAWSEc2TransitGateway_disappears
=== PAUSE TestAccAWSEc2TransitGateway_disappears
=== RUN   TestAccAWSEc2TransitGateway_AmazonSideASN
=== PAUSE TestAccAWSEc2TransitGateway_AmazonSideASN
=== RUN   TestAccAWSEc2TransitGateway_AutoAcceptSharedAttachments
=== PAUSE TestAccAWSEc2TransitGateway_AutoAcceptSharedAttachments
=== RUN   TestAccAWSEc2TransitGateway_DefaultRouteTableAssociationAndPropagationDisabled
=== PAUSE TestAccAWSEc2TransitGateway_DefaultRouteTableAssociationAndPropagationDisabled
=== RUN   TestAccAWSEc2TransitGateway_DefaultRouteTableAssociation
=== PAUSE TestAccAWSEc2TransitGateway_DefaultRouteTableAssociation
=== RUN   TestAccAWSEc2TransitGateway_DefaultRouteTablePropagation
=== PAUSE TestAccAWSEc2TransitGateway_DefaultRouteTablePropagation
=== RUN   TestAccAWSEc2TransitGateway_DnsSupport
=== PAUSE TestAccAWSEc2TransitGateway_DnsSupport
=== RUN   TestAccAWSEc2TransitGateway_VpnEcmpSupport
=== PAUSE TestAccAWSEc2TransitGateway_VpnEcmpSupport
=== RUN   TestAccAWSEc2TransitGateway_Description
=== PAUSE TestAccAWSEc2TransitGateway_Description
=== RUN   TestAccAWSEc2TransitGateway_Tags
=== PAUSE TestAccAWSEc2TransitGateway_Tags
=== CONT  TestAccAWSEc2TransitGateway_DefaultRouteTablePropagation
=== CONT  TestAccAWSEc2TransitGateway_AutoAcceptSharedAttachments
=== CONT  TestAccAWSEc2TransitGateway_Tags
=== CONT  TestAccAWSEc2TransitGateway_Description
--- PASS: TestAccAWSEc2TransitGateway_Description (250.44s)
=== CONT  TestAccAWSEc2TransitGateway_VpnEcmpSupport
--- PASS: TestAccAWSEc2TransitGateway_AutoAcceptSharedAttachments (250.46s)
=== CONT  TestAccAWSEc2TransitGateway_DnsSupport
--- PASS: TestAccAWSEc2TransitGateway_Tags (250.75s)
=== CONT  TestAccAWSEc2TransitGateway_DefaultRouteTableAssociation
--- PASS: TestAccAWSEc2TransitGateway_DefaultRouteTablePropagation (458.23s)
=== CONT  TestAccAWSEc2TransitGateway_AmazonSideASN
--- PASS: TestAccAWSEc2TransitGateway_DnsSupport (244.72s)
=== CONT  TestAccAWSEc2TransitGateway_disappears
--- PASS: TestAccAWSEc2TransitGateway_VpnEcmpSupport (245.00s)
=== CONT  TestAccAWSEc2TransitGateway_DefaultRouteTableAssociationAndPropagationDisabled
--- PASS: TestAccAWSEc2TransitGateway_DefaultRouteTableAssociationAndPropagationDisabled (154.88s)
=== CONT  TestAccAWSEc2TransitGateway_basic
--- PASS: TestAccAWSEc2TransitGateway_DefaultRouteTableAssociation (443.14s)
=== CONT  TestAccAWSEc2TransitGateway_disappears
    resource_aws_ec2_transit_gateway_test.go:151: [INFO] Got non-empty plan, as expected
--- PASS: TestAccAWSEc2TransitGateway_disappears (200.45s)
--- PASS: TestAccAWSEc2TransitGateway_basic (204.20s)
--- PASS: TestAccAWSEc2TransitGateway_AmazonSideASN (453.35s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	913.648s
...

To set either (default_route_table_association|default_route_table_propagation) you must pass in a route table ID.
On creation, if either of (default_route_table_association|default_route_table_propagation) are set to enable, a default route table is created, and the ID of that route table is used.
On modification, a route table is not created, so an ID must be passed in manually.

Hoping for some guidance, so far I left enabling either of those 2 out of this PR (although now disabling them won't ForceNew), but if the consensus is that they should be fully included, I suppose we could create a routeTable as part of the Update function if any of these change to "enable". Note that making this same change via console would require you to first create a TGW route table.

[bflad]

This looks great, thank you especially for all the testing additions 🚀

With regards for default_route_table_(association|propagation) changes without the default route table existing, we can punt on that for now here and potentially document or add a followup change set to handle/error for that situation.

Output from acceptance testing:

--- PASS: TestAccAWSEc2TransitGateway_AmazonSideASN (491.82s)
--- PASS: TestAccAWSEc2TransitGateway_AutoAcceptSharedAttachments (249.04s)
--- PASS: TestAccAWSEc2TransitGateway_basic (249.90s)
--- PASS: TestAccAWSEc2TransitGateway_DefaultRouteTableAssociation (490.64s)
--- PASS: TestAccAWSEc2TransitGateway_DefaultRouteTableAssociationAndPropagationDisabled (173.44s)
--- PASS: TestAccAWSEc2TransitGateway_DefaultRouteTablePropagation (490.24s)
--- PASS: TestAccAWSEc2TransitGateway_Description (249.05s)
--- PASS: TestAccAWSEc2TransitGateway_disappears (254.64s)
--- PASS: TestAccAWSEc2TransitGateway_DnsSupport (248.99s)
--- PASS: TestAccAWSEc2TransitGateway_Tags (247.02s)
--- PASS: TestAccAWSEc2TransitGateway_VpnEcmpSupport (248.35s)

[ghost]

This has been released in version 3.15.0 of the Terraform AWS provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template for triage. Thanks!

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you feel this issue should be reopened, we encourage creating a new issue linking back to this one for added context. Thanks!