Add DidConfirmation to list of confirmation methods

[msporny]

The specification currently only defines VerificationKeyConfirmation as a confidence method. We should also add DidConfirmation as a confidence method such that key rotation is possible.

[jandrieu]

Closing as duplicate of #20, with a note that we don't yet actually have any confidence methods described.

The spec has an example with VerificationKeyConfirmation, but not a description of what that means.