feat: add metadata-agregator gRPC client integration

- Add tonic/prost for gRPC client generation
- Add proto definitions from metadata-agregator service
- Add MetadataClient and MetadataService for gRPC communication
- Add REST controller exposing metadata endpoints (search, get, sync)
- Update config with metadata.endpoint setting
- Update flake.nix with protobuf for proto compilation

Cargo.toml

@@ -23,6 +23,11 @@ thiserror = "2"
 url = "2"
 roxmltree = "0.20"
 base64 = "0.22"
+tonic = "0.12"
+prost = "0.13"
+
+[build-dependencies]
+tonic-build = "0.12"
 
 [profile.release]
 opt-level = 3