Add schema for Cloud Foundry application manifests (manifest.yml)

[jameshochadel]

This PR adds, registers, and tests a schema for Cloud Foundry application manifests. Cloud Foundry is an open-source platform-as-a-service system in use in many organizations.

Reference material for the schema includes:

• Cloud Foundry Documentation: App manifest attribute reference
• Cloud Foundry v3 API reference, Manifest Schema

Where the two conflicted, I did my best to include the right information. I may follow up with another PR after testing manifest with my Cloud Foundry instance to see what the API server actually accepts.

[github-actions[bot]]

Thanks for the PR!

This section of the codebase is owned by @madskristensen and @hyperupcall - if they write a comment saying "LGTM" then it will be merged.

[madskristensen]

Thanks

[jameshochadel]

Thank you!