feat: [c daemon] verify_envelope_signature

[realvarx]

- What I did

• verification of sshnp-to-sshnpd request notification payloads

- How I did it

• verify_envelope_signature function

- How to verify it Run sshnpd normally with verbose flag. A new DEBG message should indicate wether the signature verification was successful.

If the original signature is modified before it is verified:

value[100] = 'A';

We will observe how the verification fails:

- Description for the changelog

[realvarx]

Thanks! I will likely move this to another file, since I need this for npt_request too.

So would you like to merge this branch, or create a different one with these commits and other new features?