FLIP-XXX: Introduce the ScalingExecutorPlugin SPI

[Dennis-Mircea]

What is the purpose of the change

(For example: This pull request adds a new feature to periodically create and maintain savepoints through the FlinkDeployment custom resource.)

Brief change log

(for example:)

• Periodic savepoint trigger is introduced to the custom resource
• The operator checks on reconciliation whether the required time has passed
• The JobManager's dispose savepoint API is used to clean up obsolete savepoints

Verifying this change

(Please pick either of the following options)

This change is a trivial rework / code cleanup without any test coverage.

(or)

This change is already covered by existing tests, such as (please describe tests).

(or)

This change added tests and can be verified as follows:

(example:)

• Added integration tests for end-to-end deployment with large payloads (100MB)
• Extended integration test for recovery after master (JobManager) failure
• Manually verified the change by running a 4 node cluster with 2 JobManagers and 4 TaskManagers, a stateful streaming program, and killing one JobManager and two TaskManagers during the execution, verifying that recovery happens correctly.

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

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

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)