Add message signature verification support

[lornajane]

We don't currently offer verification for message signatures, this should be added and users would like it (see also https://twitter.com/amlwwalker/status/1199999132758355968)

[lornajane]

🎉 check out this sample code https://gist.github.com/amlwwalker/3213e01424364425748446e43542d5b6 thanks @amlwwalker

[sasha27]

Hi @lornajane has this already been worked on? I would like to take a look if not for Hacktoberfest?

[lornajane]

I don't think anyone is working on it yet, so be my guest! Check the docs https://developer.nexmo.com/concepts/guides/signing-messages and let us know if you have questions