Pull Requests: Add merge endpoint

[N-o-Z]

Closes #8227

Change Description

Add an endpoint for pulls which will handle merge of pull requests

Testing Details

Added controller unit tests

[github-actions[bot]]

:recycle: PR Preview b1db9466db06a2820563cb596806cbd2f9fb73a6 has been successfully destroyed since this PR has been closed.

_{🤖 By surge-preview}

[github-actions[bot]]

E2E Test Results - DynamoDB Local - Local Block Adapter

[github-actions[bot]]

E2E Test Results - Quickstart