Add apiTlsPolicy trait and OpenAPI mapper

[mee-aa]

API Gateway REST APIs have a TLS security policy and an optional endpoint access mode that control the TLS version, cipher suite, and how the API endpoint can be accessed. These were previously only configurable via OpenAPI extensions.

Background

• What do these changes do?
  • Add a new apiTlsPolicy structure trait to smithy-aws-apigateway-traits with a required securityPolicy string field and an optional endpointAccessMode enum field (BASIC or STRICT).
  • Add the corresponding OpenAPI mapper in smithy-aws-apigateway-openapi that writes securityPolicy to the x-amazon-apigateway-security-policy extension and endpointAccessMode to the x-amazon-apigateway-endpoint-access-mode extension as separate top-level entries, matching how API Gateway reads them at import time.
  • Add trait documentation in amazon-apigateway.rst and the OpenAPI conversion guide in converting-to-openapi.rst.
• Why are they important?
  • Without this trait, customers had to use jsonAdd workarounds in smithy-build.json to configure TLS policies and endpoint access modes.
  • securityPolicy uses String (not an enum) because TLS policy values are actively expanding (e.g., SecurityPolicy_TLS13_* variants). endpointAccessMode uses an enum since only BASIC and STRICT are supported and unlikely to change.

Testing

• Unit tests for the trait class: builder construction with all fields, required-field-only (optional absent), round-trip via Provider, factory registration
• Test values varied across tests: legacy policies (TLS_1_0, TLS_1_2), enhanced policies (SecurityPolicy_TLS13_1_3_2025_09, SecurityPolicy_TLS13_1_2_2021_06), and both access modes (BASIC, STRICT)
• Integration test for the OpenAPI mapper: loads a Smithy model with the trait applied, converts to OpenAPI, and verifies both extensions are present with correct values
• All existing tests pass

Links

• [x-amazon-apigateway-security-policy](https://docs.aws.amazon.com/apigateway/latest/developerguide/openapi-extensions-security-policy.html)
• [x-amazon-apigateway-endpoint-access-mode](https://docs.aws.amazon.com/apigateway/latest/developerguide/openapi-extensions-endpoint-access-mode.html)
• [Security policies for REST APIs](https://docs.aws.amazon.com/apigateway/latest/developerguide/apigateway-security-policies.html)

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

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