Allow signing arbitrary data

[noandrea]

• I'm submitting a ...

  • [ ] bug report
  • [x] feature request
  • [ ] support request => Please do not submit support request here, see note at the top of this template.

• What is the current behavior?

currently the cli supports only signing transaction string tx_yaddayadda including the network-id

• What is the expected behavior?

exposes the basic signing functionality and allow to sign arbitrary data with it

• Please tell us about your environment:

  • SDK Version: v5.0.0
  • Python version: v3.7.0

• Other information (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. forum, telegram, etc)

[noandrea]

Signin arbitrary data is based on EIP712 but with prefix: b'\x19Aeternity Signed Message:...