Add authentication_method and service_access_role_arn to postgres_settings for aws_dms_endpoint

[dacohen]

Rollback Plan

If a change needs to be reverted, we will publish an updated version of the library.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

Description

This PR adds the authentication_method and service_access_role_arn settings to the postgres_settings block for aws_dms_endpoint. These are necessary for using IAM authentication with a DMS Endpoint.

Relations

Closes #43289

References

Output from Acceptance Testing

% make testacc TESTS=TestAccDMSEndpoint_PostgreSQL_settings PKG=dms

make: Verifying source code with gofmt...
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go1.24.5 test ./internal/service/dms/... -v -count 1 -parallel 20 -run='TestAccDMSEndpoint_PostgreSQL_settings'  -timeout 360m -vet=off
2025/07/17 23:15:15 Creating Terraform AWS Provider (SDKv2-style)...
2025/07/17 23:15:15 Initializing Terraform AWS Provider (SDKv2-style)...
=== RUN   TestAccDMSEndpoint_PostgreSQL_settings_source
=== PAUSE TestAccDMSEndpoint_PostgreSQL_settings_source
=== RUN   TestAccDMSEndpoint_PostgreSQL_settings_target
=== PAUSE TestAccDMSEndpoint_PostgreSQL_settings_target
=== CONT  TestAccDMSEndpoint_PostgreSQL_settings_source
=== CONT  TestAccDMSEndpoint_PostgreSQL_settings_target
--- PASS: TestAccDMSEndpoint_PostgreSQL_settings_target (123.34s)
--- PASS: TestAccDMSEndpoint_PostgreSQL_settings_source (123.71s)
PASS
ok      github.com/hashicorp/terraform-provider-aws/internal/service/dms        128.146s

[github-actions]

✅ Thank you for correcting the previously detected issues! The maintainers appreciate your efforts to make the review process as smooth as possible.

[github-actions]

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