feat(xdbg): basic send message command

[insipx]

adds

cargo xdbg send message "sup" 844301cdf88e75d841105684f782c5bc

to send a message using a random user to a specific group

This is useful for inspecting network payloads in-flight, for instance running this curl command with libxmtp in http-api mode:

curl -H 'Content-Type: application/json' \-d '{  "filters": [    {      "groupId": "Bbus/1pm8uOzvmRk/Cp6LA=="    }  ]}' -X POST http://localhost:5555/mls/v1/subscribe-group-messages

and running the xdbg send-message command. Used to help debug #997 for browser streaming