grpc: Add grpc service support (A102)

[mbissa]

This PR adds support for grpc service as part of A102

RELEASE NOTES:

• xds: Add grpc service support (A102)

[codecov]

Codecov Report

❌ Patch coverage is 57.56757% with 157 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.93%. Comparing base (cd25711) to head (1a1163b).
⚠️ Report is 16 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/xds/xdsclient/xdsresource/grpc_service.go	58.16%	44 Missing and 20 partials ⚠️
internal/xds/bootstrap/bootstrap.go	63.09%	22 Missing and 9 partials ⚠️
internal/xds/xdsclient/clientimpl.go	0.00%	30 Missing ⚠️
internal/xds/httpfilter/headers.go	65.51%	15 Missing and 5 partials ⚠️
internal/xds/resolver/xds_resolver.go	46.15%	3 Missing and 4 partials ⚠️
internal/xds/httpfilter/extproc/ext_proc.go	85.00%	2 Missing and 1 partial ⚠️
internal/xds/xdsclient/pool.go	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9175      +/-   ##
==========================================
- Coverage   83.20%   82.93%   -0.27%     
==========================================
  Files         418      420       +2     
  Lines       33741    34090     +349     
==========================================
+ Hits        28073    28272     +199     
- Misses       4250     4364     +114     
- Partials     1418     1454      +36     

Files with missing lines	Coverage Δ
internal/xds/httpfilter/httpfilter.go	87.50% <ø> (ø)
internal/xds/xdsclient/client.go	100.00% <ø> (ø)
...nternal/xds/xdsclient/xdsresource/unmarshal_lds.go	93.62% <100.00%> (+0.05%)	⬆️
internal/xds/xdsclient/pool.go	82.30% <60.00%> (-1.04%)	⬇️
internal/xds/httpfilter/extproc/ext_proc.go	84.92% <85.00%> (+8.39%)	⬆️
internal/xds/resolver/xds_resolver.go	85.66% <46.15%> (-1.17%)	⬇️
internal/xds/httpfilter/headers.go	65.51% <65.51%> (ø)
internal/xds/xdsclient/clientimpl.go	63.52% <0.00%> (-14.78%)	⬇️
internal/xds/bootstrap/bootstrap.go	63.76% <63.09%> (-0.18%)	⬇️
internal/xds/xdsclient/xdsresource/grpc_service.go	58.16% <58.16%> (ø)

... and 25 files with indirect coverage changes

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[mbissa]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces support for whitelisting allowed gRPC services (allowed_grpc_services) in the xDS bootstrap configuration, enabling secure connections to external processing servers with specified channel and call credentials. It also adds a HeaderMutator utility to compile and apply header mutations on gRPC metadata. The review feedback highlights several critical issues, including potential resource leaks in AllowedGrpcService.UnmarshalJSON if credential building fails midway, and multiple potential nil pointer dereferences when handling allowed gRPC services or credential plugins. Additionally, the reviewer recommends caching the bootstrap configuration in parseGRPCServiceConfig to avoid highly inefficient, repeated file I/O and JSON parsing.

[easwars]

I'd like to request that this PR be split into smaller PRs:

• Changes to the xDS credentials registry to support configuring credentials using proto messages.
  • I currently don't see changes in this PR at all
• Implementation of allowed_grpc_services field in the bootstrap config.
• Implementation of header mutations.
• Implementation of parsing the GrpcService proto.

Some of these PRs can be in review in parallel.