Split CLI to subcommands (and migrate to picocli)

[oleg-nenashev]

Plugin Installation Manager Tool implements multiple commands at the moment, and options are a bit confusing. The tool would benefit from sub-commands (install-plugins, list-updates) that could simplify the interface for users and allow to easily extend it going forward.

Such a migration would also allow using Plugin Installation Manager as a subcommand on its own, e.g. inside Jenkinsfile Runner CLI which needs plugin management.

FTR https://groups.google.com/g/jenkinsci-dev/c/jTkOpdZ7z1s for the discussion. Reference migration: jenkinsci/jenkinsfile-runner#432

[oleg-nenashev]

@timja @stopalopa @jetersen I have started a PoC for this change: #376 Will try to finalize it for reviews in a few days. Are we all onboard with a breaking change and Plugin Installation Manager 3.0?

[jetersen]

I am okay with breaking change especially because it allows for easier integration of new features.

[timja]

Breaking changes are fine here, ideally have an upgrade guide / command map table though.

I would just include in PR and release notes, maybe README can link to it (not a huge fan of leaving huge upgrade guides in READMEs)