feat: add GetCommits endpoint for efficient multiple commits fetching

[marcsanmi]

This PR introduces a significant enhancement to the VCS service by adding a new endpoint for parallel fetching of multiple commits. This change improves performance and efficiency when retrieving commit information for multiple refs while maintaining full backwards compatibility

Key changes

• Added a new GetCommits endpoint in the proto definition to handle multiple refs in a single request
• Implemented a new getCommits function that fetches commits in parallel using goroutines and channels
• Updated the service layer to handle partial successes and provide detailed logging for the new endpoint
• Added comprehensive tests for the new functionality
• Maintained the existing GetCommit endpoint for full backwards compatibility

Main benefits

1. Reduced network overhead: The frontend can now request multiple commits in a single API call, significantly reducing the number of HTTP requests.
2. Improved performance: Parallel fetching of commits leads to faster overall response times, especially when dealing with multiple refs.
3. Better error handling: The service now provides detailed information about both successful and failed commit fetches.
4. Backwards compatibility: Existing integrations using the GetCommit endpoint remain fully functional and unchanged.

Frontend impact

• No breaking changes for existing frontend code
• Frontend can update their code to use the new GetCommits endpoint to fetch multiple commits at once.