[FLINK-36451][runtime] Replaces LeaderElection#hasLeadership with LeaderElection#runAsLeader

[XComp]

What is the purpose of the change

See reasoning in FLINK-36451 comment

Brief change log

• Adds runAsLeader method that executes callbacks on the leadership main thread
• Replaces hasLeadership calls with runAsLeader

Verifying this change

• Adds unit tests
• Rewrites hasLeadership tests in unit test classes

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 changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: yes
• The S3 file system connector: no

Documentation

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

[flinkbot]

CI report:

• 91eb73c Azure: FAILURE

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build