Add core JSON-RPC 2.0 protocol implementation for Kanboard API:
- JSONRPCRequest/Response/Error structs with proper JSON tags
- Generic call() method for sending requests and parsing responses
- Thread-safe request ID generation using atomic.Int64
- Automatic /jsonrpc.php path appending to baseURL
- Support for subdirectory installations
- HTTP Basic Auth support (API token and username/password)
- Error handling for unauthorized/forbidden responses
Includes comprehensive tests with httptest mock server.
Closes: kanboard-api-2g1
