Make JCasC configuration source extensible

[oleg-nenashev]

Currently the JCasC plugin supports loading configurations only from the filesystem. At the same time, there are use-cases when it is more convenient to use alternate source. It would be great to make the configuration sources extensible and somehow configurable in Jenkins.

Examples of external sources:

• SCM, e.g. Git - git + branch as configuration source #273
• S3 buckets
• Kubernetes CRD (especially when used with the Jenkins Kubernetes Operator)

This story is somewhat related to the Configuration Pluggable Storage. I would argue that implementation of that on the JCasC side is enough for immutable images, and that it might close the use-case for "cloud Native Jenkins".

[timja]

Isn't this: #34

[oleg-nenashev]

Yes, it is #34. Missed it (likely due to the non-obvious name)

[jetersen]

I think we would be best served with having the Configuration source be extensible and that configuration-as-code exposes an extension and we would also provide said extension in a separate plugin. That could depend on the varies sources, depending on git plugin for instance.

[jetersen]

We could even move the existing configuration sources to an internal extension to keep compatibility