xds: authority rewrite literal

[zhiyanfoo]

This implements respecting the host_rewrite route action for grpc-xds clients. The grpc-proposal for this issue is here.

There was also an initial discussion here

RELEASE NOTES:

• xds: Adds support for host_rewrite route action for grpc-xds clients. gRFC A111

[codecov]

Codecov Report

❌ Patch coverage is 92.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.29%. Comparing base (25dbb81) to head (e991c20).
⚠️ Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/testutils/balancer.go	80.00%	0 Missing and 1 partial ⚠️
internal/xds/balancer/clusterimpl/picker.go	92.30%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8838   +/-   ##
=======================================
  Coverage   83.29%   83.29%           
=======================================
  Files         414      414           
  Lines       32753    32777   +24     
=======================================
+ Hits        27280    27301   +21     
  Misses       4070     4070           
- Partials     1403     1406    +3     

Files with missing lines	Coverage Δ
internal/envconfig/envconfig.go	100.00% <ø> (ø)
internal/xds/resolver/serviceconfig.go	88.32% <100.00%> (+0.17%)	⬆️
internal/xds/resolver/xds_resolver.go	88.82% <100.00%> (+0.06%)	⬆️
...nternal/xds/xdsclient/xdsresource/unmarshal_rds.go	87.84% <100.00%> (+0.09%)	⬆️
internal/testutils/balancer.go	84.09% <80.00%> (+1.80%)	⬆️
internal/xds/balancer/clusterimpl/picker.go	93.47% <92.30%> (-1.65%)	⬇️

... and 21 files with indirect coverage changes

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[atollena]

Closing for now, as grpc/proposal#533 needs to be approved first.