[FLINK-31857] Observer plugin

[darenwkt]

What is the purpose of the change

Support pluginable Observer to allow user to load a plugin JAR to run custom observer logic.

Brief change log

• Added FlinkResourceObserverPlugin interface for users to implement in plugin JAR with custom observer logic.

Verifying this change

• Added unit test to verify TestObserverPlugin is discoverable.
• Manual test by loading a sample JAR that changes the jobName in FlinkSessionJob status field and verified successfully.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): ( no)
• The public API, i.e., is any changes to the CustomResourceDescriptors: ( no)
• Core observer or reconciler logic that is regularly executed: (yes)

Documentation

• Does this pull request introduce a new feature? (yes)
• If yes, how is the feature documented? (docs)

[gyfora]

I think we should design this together with the reconciler plugin and there should be a FLIP for it to make sure that other users can influence the design and we cover existing features like the autoscaler etc with the pluggable mechanism.