To test the grpc interface we'll want to emulate the JSON-RPC
interface as best we can, hence when talking to the grpc interface we
want to convert back into a parsed JSON format as LightningRpc would
have returned it. This is just the simplest way of closing the loop
here:
```
pyln-testing --grpc-> cln-grpc --grpc2json
^ |
| v
| JSON-RPC
| |
TEST v
^ CLN
| |
| v
pyln-testing <-grpc2py-- cln-grpc <- json2grpc
```
