Handle BOOKMARK events in watch.py

[note35]

/kind feature

What this PR does / why we need it:

This PR implements the TODO in watch.py to handle BOOKMARK events. Previously, BOOKMARK events were ignored.

Changes included:

1. Updated unmarshal_event to extract and update resource_version when receiving BOOKMARK events.
2. Refactored unmarshal_event using guard clauses to improve readability and reduce nesting.
3. Added unit tests to verify bookmark handling.

Which issue(s) this PR fixes:

It fixes TODO in kubernetes/watch/watch.py, I think no issue is created for this.

Special notes for your reviewer:

Verified with python -m unittest kubernetes/watch/watch_test.py
Applied flake8 and isort on impacted lines. (No <= 80 characters limitation.)

Does this PR introduce a user-facing change?

Optimized Watch client to handle BOOKMARK events' resource version.

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

N/A

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

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