xds/resolver: Optimize Interceptor Chain Construction

[easwars]

Existing behavior:

• At routing time, when an RPC matches a route and a cluster is selected, the interceptor chain for that specific RPC is built.
• This chain is built on a per-RPC basis.
• A subsequent RPC that matches the exact same route and cluster will trigger the entire chain reconstruction again, even if no configuration has changed.

New behavior:

• The interceptor chain is now pre-built for every route and every pickable cluster associated with that route.
• The chains are constructed once when the config selector is built.

Other changes:

• Existing unit tests have been converted to be more e2e style tests.
• This lays the necessary groundwork for upcoming changes to the filter API, specifically to support filter state retention

RELEASE NOTES: NONE

[easwars]

@eshitachandwani : FYI this might conflict with your resolver changes for A74. And since you have been looking at the resolver code for sometime now, it would be a good PR to review.

[codecov]

Codecov Report

❌ Patch coverage is 46.87500% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.09%. Comparing base (8110884) to head (226e890).
⚠️ Report is 31 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/xds/resolver/serviceconfig.go	45.00%	4 Missing and 7 partials ⚠️
internal/xds/resolver/xds_resolver.go	50.00%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8641      +/-   ##
==========================================
+ Coverage   79.45%   83.09%   +3.64%     
==========================================
  Files         415      415              
  Lines       41339    32144    -9195     
==========================================
- Hits        32844    26710    -6134     
+ Misses       6621     4021    -2600     
+ Partials     1874     1413     -461     

Files with missing lines	Coverage Δ
internal/xds/resolver/xds_resolver.go	73.64% <50.00%> (-8.13%)	⬇️
internal/xds/resolver/serviceconfig.go	65.03% <45.00%> (-20.53%)	⬇️

... and 365 files with indirect coverage changes

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[dfawley]

@arjan-bal could you review this change, please?

[arjan-bal]

Leaving my comments on the non-test code, still reviewing the tests.

[easwars]

@arjan-bal : Gentle ping on this one. This is blocking a design change that I need to make to support A83.

[arjan-bal]

LGTM!