Expose Version Control REST API endpoints to allow for CI automation

[yaustar]

For some workflows where developers have code in separate version control repo, having the API endpoints would really help with automation of workflows.

• Create checkpoints
• Get checkpoints from a branch
• Create branches
• Get checkpoint metadata (message, etc)
• Get checkpoint changed files
• Merge branches - option to merge if there are no conflicts
• Close/Open branches
• Switch branches

[yaustar]

From #539

• Fetch list of projects with ids
• Authenticate a user and get an access token in return
• Make checkpoints
• Switch branches
• Merge branches

[yaustar]

Related thread: https://forum.playcanvas.com/t/switching-branches-via-nodejs/22446

[yaustar]

Thinking on this a little more:

• Create checkpoints
• Switch branches
• Merge branches - option to merge if there are no conflicts
• Create branches
• Close/Open branches
• Get checkpoints from a branch
• Get checkpoint data (message, changed files etc)

Is there anyway we can do a merge preview where a 'diff' could be downloaded? Could we utilise the PlayCanvas Editor diff viewer to be able to load the diff file directly?