Permit DIDCommService type to be a list?

[dbluhm]

An aspect of services that was originally overlooked in this implementation was that the type of a service may be either a string or a list of strings. #82 corrected the base Service class to permit this; however, DIDCommService classes are still strictly expecting to be just a string. Should we update this to accept a list of strings as well?

Our current posture is to wait until this is a problem since it will require some changes to how DIDComm Services are parsed.

Thank you, @PatStLouis, for bringing the original issue up.