[FLINK-13960] Add throwing HighAvailabilityServices#getWebMonitorLeaderRetriever default impl

[tillrohrmann]

What is the purpose of the change

Default impl HighAvailabilitySerivces.getWebMonitorLeaderRetriever throws exception referring to implement the ClientHighAvailabilityServices instead.

cc @tisonkun

Verifying this change

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

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• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
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• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (no)
• 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)

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[tisonkun]

LGTM +1

To be clear, as discussed in FLINK-13964 we might introduce a getRestEndpointRetriever in HighAvailabilityServices with inheriting ClientHighAvailabilityServices whose default implementation is delegate to getWebMonitorLeaderRetriever. Is it correct?

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CI report:

• 7ef0929 : SUCCESS Build

[tillrohrmann]

Yes this is correct. Thanks for your review @tisonkun.

[tillrohrmann]

Thanks for the review @tisonkun. Merging this PR.