extproc: implement ClientFilter and ClientFilterBuilder interface

[eshitachandwani]

This PR is part of implementing A93: xds-ext-proc

This PR adds the builder that implements the ClientFilter and ClientFilterBuilder interface. That includes creating the interceptor config from the base and the override config. It also includes making the ext_proc channel.

THis PR has a placeholder function for converting grpcService to ServerConfig proto. The function will be implemented later when implementing gRFC A102.

The interceptor struct has a resolver.ClientInterceptor embedded for now, but that will be removed in a later PR when we implement the resolver.ClientInterceptor interface. Since the filter will not be registered and no one is calling the resolver.ClientInterceptor function fro extproc filter , it will not panic.

#ext-proc-a93

RELEASE NOTES: None

[codecov]

Codecov Report

❌ Patch coverage is 90.69767% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.07%. Comparing base (06fc26a) to head (87fb4c7).
⚠️ Report is 12 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/xds/httpfilter/extproc/ext_proc.go	86.66%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9086      +/-   ##
==========================================
+ Coverage   80.52%   83.07%   +2.54%     
==========================================
  Files         413      415       +2     
  Lines       33543    33527      -16     
==========================================
+ Hits        27012    27854     +842     
+ Misses       4316     4243      -73     
+ Partials     2215     1430     -785     

Files with missing lines	Coverage Δ
internal/xds/httpfilter/extproc/config.go	100.00% <100.00%> (ø)
internal/xds/httpfilter/extproc/ext_proc.go	86.66% <86.66%> (ø)

... and 38 files with indirect coverage changes

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[eshitachandwani]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces the Envoy external processing filter, providing configuration parsing logic and the infrastructure to build a client interceptor. Feedback addresses a critical uninitialized function variable that would cause a panic during filter construction and suggests a performance optimization for slice allocation when converting string matchers.