Add message signing and verification (#45)

* Add cose dependency for message signing

* Add message class for message signing

* Add cose library for message signing

* Add tests for Message signing and verification

* Lint imports

* Reformat Message class

* Update poetry lock with cose

* Revert key module

* Move message signing to cip30 module

* Update test

* Lint CIP-30 tests

* Rename CIP30 to CIP8 and add docstrings

* Remove old print statement

* Specify CIP in docstring

Author: astrojarred