xds/resolver: pass route's auto_host_rewrite to LB picker (gRFC A81)

[Pranjali-2501]

This PR implements the ConfigSelector changes required for gRFC A81.

It ensures that the auto_host_rewrite field from the xDS Route Configuration is correctly propagated through the resolver and made available to the Load Balancer picker via the RPC context.

Key Changes:

• Pass the AutoHostRewrite field value from Route struct via RPC context.
• Add helper functions for AutoHostRewrite in internal/xds/balancer/cluserimpl/picker.go.
• Update ConfigSelector.SelectConfig to pass the AutoHostRewrite boolean in RPC context.

RELEASE NOTES: None

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.30%. Comparing base (4c27cc8) to head (29dd0bb).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8740      +/-   ##
==========================================
+ Coverage   83.28%   83.30%   +0.01%     
==========================================
  Files         418      418              
  Lines       32367    32394      +27     
==========================================
+ Hits        26958    26985      +27     
+ Misses       4034     4030       -4     
- Partials     1375     1379       +4     

Files with missing lines	Coverage Δ
internal/xds/balancer/clusterimpl/picker.go	97.40% <100.00%> (+0.25%)	⬆️
internal/xds/resolver/serviceconfig.go	88.05% <100.00%> (+0.08%)	⬆️
internal/xds/resolver/xds_resolver.go	88.63% <100.00%> (+0.06%)	⬆️

... and 22 files with indirect coverage changes

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[easwars]

Sorry for suggesting that we could possibly send the whole route instead of sending only the auto_host_rewrite field, as specified in A60. After reading A60, I realized that sending the whole route down is not as simple as just stashing a pointer to the route in an attribute. We need to take into account things like reference counts to the route and the selected cluster and the whole cluster map.

So, let's keep it simple for now and just add an attribute that carries the auto_host_rewrite just like how we pass the selected cluster name from the config selector to the LB policy. Thanks.

[easwars]

Also please remove any mentions to MatchedRoute from the PR description and from comments in the PR.

[easwars]

LGTM, modulo one minor comment

[easwars]

Nit: maybe rename the variable with a one letter name. Something like v?

See: go/go-style/decisions#variable-names

TL;Dr
The general rule of thumb is that the length of a name should be proportional to the size of its scope and inversely proportional to the number of times that it is used within that scope

[easwars]

FYI: Please read this section: go/go-style/decisions#literal-zero-value-fields

IMO, it makes sense to have the values be explicitly specified in the table for this test case (as you have done currently). So, I'm not requesting changes, but just bringing this to your attention.