FR: signing/verifying a message

[Maxnflaxl]

Feature title:

Signing/verifying a message

Technical implementation:

@valdok

Describe the solution you'd like

sign a message with CLI:
./beam-wallet sign_message --address=<address> --message=<message>
? If the --address argument is missing, use default address.
? If the --message argument is missing, only want to prove ownership of an address.
=> returns signature

verify a message with CLI:
./beam-wallet verify_message --address=<address> --message=<message> --signature=<signature>
=> successful: Good signature from <address>
=> fail: Invalid signature from <address>

sign a message with wallet-api:

-->

{
    "jsonrpc":"2.0", 
    "id": <TBD>,
    "method":"sign_message",
    "params":
    {
        "address" : "",
        "message" : "",
    }
}

<--

{
    "jsonrpc":"2.0", 
    "id": <TBD>,
    "result":
    {
        "signature" : ""
    }
}

verify a message with wallet-api:

-->

{
    "jsonrpc":"2.0", 
    "id": <TBD>,
    "method":"verify_message",
    "params":
    {
        "address" : "",
        "message" : "",
        "signature": ""
    }
}

<--

{
    "jsonrpc":"2.0", 
    "id": <TBD>,
    "result":
    {
        "signature" : ""
    }
}

Additional context
BeamMW/beam-ui#1066
https://github.com/BeamMW/beam/wiki/Beam-wallet-protocol-API-v7.3
https://beam.mw/docs/cli