[Feature] add flag to make the full output machine-readable, not only the message

Currently only the message is formatted as json, when you set `--format json`.

desired output example:
```
grpcurl --print-output-machine-readable ...
# the output:
{
"response-headers": [
{"content-type": "application/grpc"}
],
"http-status-code": 200,
"response-message": {
"field1": 123
},
"response-message-bytes": "base64string=="
}
```

Start by locating the existing --format json handling and the code that assembles grpcurl's full output. Trace how response headers, HTTP status, response messages, and response bytes are currently produced. Done means the new flag emits the complete output in the structure shown while preserving existing behavior.

Written by the indexing model from the issue text.