Cloud Composer connection subnetwork

[kubasieron]

Add support for Private Service Connect by adding cloud_composer_connection_subnetwork field.

Fixes: hashicorp/terraform-provider-google#10550

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

composer: added support for Private Service Connect by adding field `cloud_composer_connection_subnetwork` in `google_composer_environment` 

[modular-magician]

Hello! I am a robot who works on Magic Modules PRs.

I have detected that you are a community contributor, so your PR will be assigned to someone with a commit-bit on this repo for initial review.

Thanks for your contribution! A human will be with you soon.

@ScottSuarez, please review this PR or find an appropriate assignee.

[kubasieron]

Please review, all blockers are removed now

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 3 files changed, 78 insertions(+), 84 deletions(-))
Terraform Beta: Diff ( 3 files changed, 149 insertions(+), 68 deletions(-))

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccApigeeEnvironmentIamBindingGenerated|TestAccApigeeEnvironmentIamMemberGenerated|TestAccCloudFunctionsFunction_vpcConnector|TestAccComposerEnvironment_ComposerV2|TestAccComposerEnvironment_composerV2PrivateServiceConnect|TestAccComputeForwardingRule_update|TestAccComputeRegionNetworkEndpointGroup_regionNetworkEndpointGroupAppengineExample|TestAccContainerCluster_regionalWithNodePool|TestAccContainerCluster_withNodePoolResize|TestAccContainerCluster_withNodePoolAutoscaling|TestAccContainerCluster_errorAutopilotLocation|TestAccContainerNodePool_withInvalidUpgradeSettings|TestAccContainerNodePool_012_ConfigModeAttr|TestAccContainerNodePool_EmptyGuestAccelerator|TestAccContainerNodePool_ephemeralStorageConfig|TestAccServiceNetworkingPeeredDNSDomain_basic|TestAccPrivatecaCertificateTemplate_BasicCertificateTemplate|TestAccPrivatecaCertificate_privatecaCertificateNoAuthorityExample You can view the result here: https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=229911

[modular-magician]

Tests failed during RECORDING mode: TestAccContainerCluster_errorAutopilotLocation|TestAccCloudFunctionsFunction_vpcConnector|TestAccPrivatecaCertificate_privatecaCertificateNoAuthorityExample|TestAccContainerNodePool_withInvalidUpgradeSettings Please fix these to complete your PR