[FLINK-37515] Basic support for Blue/Green deployments

[schongloo]

What is the purpose of the change

This pull request adds basic support for Blue/Green deployments as outlined by FLIP-503 (https://cwiki.apache.org/confluence/pages/viewpage.action?pageId=337677648).

Brief change log

• A new FlinkBlueGreenDeploymentController controller, with the capability of managing the lifecycle of these deployments.
• A new corresponding CRD is introduced
• The new FlinkBlueGreenDeploymentController will manage this CRD and hide from the user the details of the actual Blue/Green (Active/StandBy) jobs.
• Delegate the lifecycle of the actual Jobs to the existing FlinkDeployment controller.

Verifying this change

• Test to verify a single basic deployment
• Test to verify a proper transition Blue -> Green
• Test to verify correct behavior when encountering an error before a transition
• Test to verify correct behavior when encountering an error during a transition

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: no

Documentation

• Does this pull request introduce a new feature? yes
• If yes, how is the feature documented? docs / JavaDocs. UML diagrams for all scenarios added under docs/content/docs

[davidradl]

I am concerned about the number of TODOs. Things like dealing with errors and timing conditions should be in place in the initial drop of code - unless we can mark this capability as beta or the like. WDYT @gyfora ?

[schongloo]

@davidradl thanks for your review/comments. I've cleaned up all the outdated/invalid TODOs which pointed me 1 missing unit test, added. At this point the few remaining TODOs do not interfere with the main functionality and are not critical/open for discussion.

[hansh0801]

👍