fix(fallback): backfill all affected objects, not only broken

[czeslavo]

What this PR does / why we need it:

Changes the backfilling algorithm to attempt to backfill all affected (broken + directly and indirectly affected) objects from the last valid state instead of only the broken ones (the ones reported by Gateway).

Also

• Passes the UseLastValidConfigForFallback flag value to sendconfig.Config (I could swear I tested that locally E2E and it was working 🤔)
• Fixes the integration test that incorrectly assumed that headers that we pass to EventuallyGETPath are the expected response headers (no, they're not - these are request headers 🤦). It was passing, but wasn't really tested what it should test.

Which issue this PR fixes:

Part of #5854.

[codecov]

Codecov Report

Attention: Patch coverage is 75.75758% with 8 lines in your changes missing coverage. Please review.

Project coverage is 74.5%. Comparing base (d62262e) to head (475cd8c).
Report is 1 commits behind head on main.

Files	Patch %	Lines
internal/dataplane/fallback/fallback.go	65.2%	5 Missing and 3 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #6172     +/-   ##
=======================================
+ Coverage   73.7%   74.5%   +0.8%     
=======================================
  Files        198     198             
  Lines      19742   19760     +18     
=======================================
+ Hits       14560   14739    +179     
+ Misses      4202    4025    -177     
- Partials     980     996     +16     

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[programmer04]

🙇