Add interceptor_configurations support to aws_bedrockagentcore_gateway

[thenom]

Closes #45331

Adds support for gateway interceptor configurations which allow custom Lambda functions to be invoked during gateway invocations at REQUEST and/or RESPONSE interception points.

Changes

• Added interceptor_configurations block to aws_bedrockagentcore_gateway resource schema
• Implemented proper expand/flatten logic for the InterceptorConfiguration union type
• Added documentation with examples
• Added acceptance test
• Supports 1-2 interceptor configurations per gateway
• Each interceptor can specify REQUEST and/or RESPONSE interception points
• Lambda function ARN configuration for interceptor execution
• Optional input configuration to pass request headers to interceptor

Checklist

• Code follows provider conventions
• Documentation updated with examples
• Acceptance test added
• Changelog entry (will add once PR number is assigned)

[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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