Implement sign_message method

[kieranroneill]

Summary

The sign_message signs an arbitrary UTF-8 string.

Acceptance criteria

Scenario 1: Provider receives a sign message request
GIVEN a Client has initialized the SDK
AND a Provider as initialized the SDK
WHEN the Client calls sign_message
THEN the Provider receives the UTF-8 string to sign
AND the Provider returns the signature

Notes

• Include documentation for "Signing A Message" in the "Clients" section
• Include documentation for "Responding To A Sign Message Request" in the "Providers" section