[feat] Java Core - add native Statsig forward proxy support

[trentusus]

Summary

This updates Java Core so the existing SpecAdapterConfig path can be used for native forward proxy support with the gRPC websocket adapter.

The main changes are:

• add typed Java constants for spec adapter types and authentication modes
• enable with_grpc in the Java FFI build path
• update Java build/test/publish flows to produce grpc-enabled native artifacts
• update Java tests to use the canonical adapter names that Rust already expects

This PR does not add a new forward-proxy-specific Java API. It keeps SpecAdapterConfig and setSpecAdapterConfigs(...) as the public surface.

Changes

• Added Java constants:
  • SpecAdapterType.DATA_STORE
  • SpecAdapterType.NETWORK_HTTP
  • SpecAdapterType.NETWORK_GRPC_WEBSOCKET
  • AuthenticationMode.NONE
  • AuthenticationMode.TLS
  • AuthenticationMode.MTLS
• Updated Java tests to use canonical adapter values instead of legacy "http" / "grpc" strings
• Added Java-side coverage for grpc-websocket config construction with TLS fields
• Added with_grpc feature forwarding in statsig-ffi
• Updated the active Java build/test/publish paths to build statsig_ffi with with_grpc
• Kept ProxyConfig unchanged; this remains generic outbound HTTP proxy config only

Verification

Successful testing completed locally:

• ./tore test java --no-docker

  • passed under Java 11

• ./tore build java --os macos --arch arm64 --skip-docker-build

  • passed
  • grpc-enabled native artifact was produced for the Java package

• Local forward proxy validation against a running proxy

  • plaintext gRPC on http://127.0.0.1:50051
  • TLS on https://127.0.0.1:50052
  • mTLS on https://127.0.0.1:50052

For the local forward proxy runtime validation, Java Core successfully:

• initialized through network_grpc_websocket
• loaded config specs from the proxy
• reported configSpecReady=true
• completed a basic evaluation successfully

Additional local validation:

• generated and validated local CA/server/client certs
• verified the TLS listener with openssl s_client
• confirmed proxy-side gRPC initialization logs during Java client startup