provider: Add environment variable support for DynamoDB, IAM, S3, STS endpoints

[gdavison]

Adds environment variables to override AWS API endpoints for parity with the S3 backend:

• TF_AWS_DYNAMODB_ENDPOINT
• TF_AWS_IAM_ENDPOINT
• TF_AWS_S3_ENDPOINT
• TF_AWS_STS_ENDPOINT

New and already deprecated environment variables (hashicorp/terraform#30479)

• AWS_DYNAMODB_ENDPOINT
• AWS_IAM_ENDPOINT
• AWS_S3_ENDPOINT
• AWS_STS_ENDPOINT

Closes #23025

Test results

$ go test ./internal/provider/... -v

=== RUN   TestExpandEndpoints
--- PASS: TestExpandEndpoints (0.00s)
=== RUN   TestEndpointMultipleKeys
--- PASS: TestEndpointMultipleKeys (0.01s)
=== RUN   TestEndpointEnvVarPrecedence
2022/02/08 17:39:13 [WARN] The environment variable "AWS_STS_ENDPOINT" is deprecated. Use "TF_AWS_STS_ENDPOINT" instead.
--- PASS: TestEndpointEnvVarPrecedence (0.01s)
PASS
ok  	github.com/hashicorp/terraform-provider-aws/internal/provider	1.052s

[ewbankkit]

LGTM 🚀.

% go test ./internal/provider  
ok  	github.com/hashicorp/terraform-provider-aws/internal/provider	1.511s

[github-actions]

This functionality has been released in v4.0.0 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. Thank you!

[github-actions]

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