RPC sign and verify messages with a space

[buffrr]

This PR adds two RPC commands, walletsignmessage and verifymessage enabling signing and verification of arbitrary binary messages. These commands prefix messages with Spaces signed message: \n magic bytes before signing and verification.

For space-cli, two commands signmessage and verifymessage are introduced, allowing signing and verification of plain text messages e.g.

space-cli signmessage @bitcoin "hello world"

and to verify:

space-cli verifymessage @bitcoin "hello world" --signature <signature>