feat: generate fallback cache snapshot excluding affected objects

[czeslavo]

What this PR does / why we need it:

• Implements a fallback cache Generator with the GenerateExcludingAffected method. It depends on a CacheGraphProvider implemented by DefaultCacheGraphProvider (formerly NewConfigGraphFromCacheStores function). It's tested in isolation in the TestGenerator_ GenerateExcludingAffected test.
• Implements ConfigGraph.SubgraphObjects capable of finding all objects depending on a broken source object and tests that in TestConfigGraph_SubgraphObjects
• Moves NewConfigGraphFromCacheStores to cache_to_graph.go and refactors it into a struct implementing CacheGraphProvider interface (also moves its tests to cache_to_graph_test.go with no modifications).

Which issue this PR fixes:

Closes #5930.

Special notes for your reviewer:

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• the CHANGELOG.md release notes have been updated to reflect any significant (and particularly user-facing) changes introduced by this PR

[codecov]

Codecov Report

Attention: Patch coverage is 60.71429% with 22 lines in your changes are missing coverage. Please review.

❗ No coverage uploaded for pull request base (main@d60e105). Click here to learn what that means.

❗ Current head 38084a7 differs from pull request most recent head 15c10b6. Consider uploading reports for the commit 15c10b6 to get more accurate results

Files	Patch %	Lines
internal/dataplane/fallback/cache_to_graph.go	50.0%	5 Missing and 5 partials ⚠️
internal/dataplane/fallback/fallback.go	55.5%	4 Missing and 4 partials ⚠️
internal/dataplane/fallback/graph.go	77.7%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##             main   #6047   +/-   ##
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  Coverage        ?   54.3%           
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  Files           ?     193           
  Lines           ?   18717           
  Branches        ?       0           
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  Hits            ?   10179           
  Misses          ?    7850           
  Partials        ?     688           

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