Simplify DIDComm unpack(), use Key instead of String

[JamesKEbert]

Refactors

• Simplifies the DIDComm unpack methods, as there previously used to be separate methods for authcrypt and anoncrypt, however under the hood they both would do authcrypt and anoncrypt unpacking, so the only difference really was that authcrypt would compare whether the sender verkey matched a supplied expected sender verkey (and provide an incorrect impression that only one strategy was being utilized). So I combined them and provided an optional arg for the expected sender verkey check.
• Additionally, this PR adjusts the usage of Keys to be passed everywhere by Key instead of as String to provide better typing and access to the methods on the Key object. I also added a method to do a match against a given KeyType that will return an error if it is not valid to reduce the overhead of validation needed in a variety of places. Coincidentally this change removes a couple of spots where we go Key -> String -> Key (which is gross).
• Unpacked message keys are now assumed to be of KeyType ED25519.

[gmulhearn]

lgtm