Feat/grpc healthcheck

[thorrester]

Pull Request

This PR adds gRPC reflection and flexible server deployment modes to the scouter server.

gRPC Reflection (scouter-tonic, scouter-server)

• Updated scouter_tonic/build.rs to emit a compiled proto file descriptor set (scouter_descriptor.bin) into OUT_DIR alongside the existing generated code, with proto file
  existence validation and a cargo:rerun-if-changed directive
• Exposed FILE_DESCRIPTOR_SET as a public constant from scouter-tonic so downstream crates don't need to know the OUT_DIR path
• Added tonic-reflection to the gRPC server, registering both the scouter service descriptors and the tonic_health descriptor — this enables tools like grpcurl to introspect
  the API and call grpc.health.v1.Health/Check without needing proto files

Flexible Serve Modes (scouter-server)

• Added a ServeMode enum (Both / Http / Grpc) and a start_server_with_mode(mode: ServeMode) function to lib.rs. The existing start_server() delegates to ServeMode::Both,
  keeping all existing test and library usage unchanged
• Added a clap-based Cli struct to main.rs with a --mode flag (default: both), allowing the binary to be deployed as HTTP-only or gRPC-only without code changes
• Added make start.server.http and make start.server.grpc targets for local development
• Updated make stop.server to also kill port 50051

Usage

./scouter-server # both (existing behaviour)
./scouter-server --mode http # HTTP only
./scouter-server --mode grpc # gRPC only (exposes grpc.health.v1.Health/Check on :50051)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.59%. Comparing base (4a64214) to head (4b58ac1).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #216   +/-   ##
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  Coverage   62.59%   62.59%           
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  Files          18       18           
  Lines         262      262           
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  Hits          164      164           
  Misses         98       98           

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