Move Java Spring and Python FastAPI apps to final dir structure

[dacharyc]

This PR sets up the final dir structure for Java Spring and Python FastAPI server backends, including:

• Move java-spring from server/java-spring to mflix/server/java-spring
• Move python from server/python to mflix/server/python-fastapi
• Update paths in GitHub CI workflows, and also have only the relevant workflow trigger on PR based on PR file paths
• Add gitignores appropriately named, at the appropriate place in the dir structure, and containing the appropriate backend + client content in prep for the copy utility
• Add the missing VOYAGE_API_KEY to Java and Python .env.example files

I intentionally did not modify anything related to the Express app here since that app is still getting wrapped up. We can do that after all the open PRs related to it are closed.

The CI workflows will fail on this PR run because GitHub CI is using the ones in the upstream repo, not the ones in this PR from my fork. Once we merge this PR, the CI workflows should work again.

[jordan-smith721]

Thanks for grabbing this! LGTM