Add JSON-RPC 2.0 Transport Support to Goa

[raphael]

This PR introduces JSON-RPC 2.0 protocol support to the Goa framework, enabling developers to build JSON-RPC APIs alongside HTTP REST and gRPC using the same DSL.

Features

Core JSON-RPC Support

• Full JSON-RPC 2.0 Compliance: Implements the complete JSON-RPC 2.0 specification including request/response format, error codes, and batch requests
• Multiple Transport Options:
  • HTTP POST for standard RPC calls
  • WebSocket for bidirectional streaming
  • Server-Sent Events (SSE) for server-side streaming
• Unified DSL: Use the same Goa DSL to define services that can be exposed via REST, gRPC, and JSON-RPC

Advanced Features

• Batch request processing: The generated servers and clients support single elements or arrays of elements in request params
• Request ID correlation: The generated code takes care of populating the request ID in responses

Code Generation

• Server Generation: Complete server implementation with routing, request decoding, and response encoding
• Client Generation: Type-safe client with automatic request/response handling
• Streaming Support: Generated code for WebSocket and SSE streaming endpoints
• Example Generation: Working server and client examples via goa example

DSL Integration

  Service("Calculator", func() {
      Method("add", func() {
          Payload(func() {
              Attribute("a", Int)
              Attribute("b", Int)
          })
          Result(Int)

          // JSON-RPC transport configuration
          JSONRPC(func() {
              POST("/rpc")  // HTTP endpoint
              MethodName("Calculator.Add")  // Custom method name
          })
      })
  })

Method Routing

JSON-RPC transport brings a key architectural difference: multiple methods can share the same HTTP endpoint, with method routing handled by the JSON-RPC method field rather than URL paths. This allows for cleaner API organization where related methods can be grouped under a single endpoint.

Service("Calculator", func() {
    Method("add", func() {
        JSONRPC(func() {
            POST("/api/v1")  // Same endpoint as subtract
        })
    })

    Method("subtract", func() {
        JSONRPC(func() {
            POST("/api/v1")  // Same endpoint as add
        })
    })

    Method("multiply", func() {
        JSONRPC(func() {
            POST("/api/v2")  // Different version endpoint
        })
    })
})

Testing

Comprehensive test coverage including protocol compliance tests, integration tests, and golden file tests for generated code. All tests pass with make.