Plug adapter for the bare JSON-RPC payment transport.

Mount on a POST route (or forward) with the same endpoint options as MPP.Plug, plus a :handler that receives the verified JSON-RPC request map and returns a result or JSON-RPC response:

forward "/rpc",
  to: MPP.Transports.JsonRpc.Plug,
  init_opts: [
    handler: &MyApp.Rpc.dispatch/1,
    secret_key: secret,
    realm: "rpc.example.com",
    method: MyMethod,
    amount: "1000",
    currency: "usd"
  ]

Payment-required and verification failures are JSON-RPC errors (-32042 / -32602 / -32043) on HTTP 200, matching JSON-RPC-over-HTTP convention and the MPP transport spec's error.data.httpStatus: 402 field. Payment-gated notifications (JSON-RPC requests with no id) are dropped with HTTP 204.