[FLINK-17634][rest] Reject multiple registration for the same endpoint

[zentol]

Adds a new check to the RestServerEndpoint to ensure that there aren't multiple registration for the same (version, method, url), which happened in FLINK-17562.

So far we only checked that a handler isn't being re-used for multiple registrations.

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[tillrohrmann]

Thanks for creating this PR @zentol. There are AZP failures which seem to be caused by this PR.

[zentol]

@tillrohrmann The branch did not include the fix for FLINK-17562, and failed accordingly. So it works at least, an the build is now green after a rebase.

[tillrohrmann]

Thanks for creating this PR @zentol. LGTM. +1 for merging this PR.