Add EdDsa algprithm to GetPublicKeyAlgorithm() method

[Gymbotan]

Hi there! Thank you for the MimeKit!

Problem description
I ran into a small problem. I've got a PGP message from another app. It contains a signature that uses EdDsa algorithm. When I try to verify the signature MimeKit throws an exception in the following method:
https://github.com/jstedfast/MimeKit/blob/master/MimeKit/Cryptography/OpenPgpContext.cs#LL1099C3-L1099C10

Possible solution
At the same time, there is exist PublicKeyAlgorithmTag.EdDsa tag and PublicKeyAlgorithm.EdwardsCurveDsa algorithm. In case they both correspond to generating/verifying an signature and work correctly, could you please add this functionality?

[jstedfast]

Ah, good. I guess this means that BouncyCastle must have recently added support for that.

Yes, I will update MimeKit to add support as well.