Traits for DID method operations

[clehner]

#379 (review)

If DIDMethodError::OptionNotSupported means that not all methods support all operations, maybe that means the trait should be split, to get compile time guarantees.

#379 adds DID operation functions to the DIDMethod trait: create, update, recover and deactivate. But these are not required to implement the trait, and default to return an error. Some DID methods don't implement any of those operations; some implement all of them, and some may implement some but not others. Create, update and deactivate are described in DID Core, although they do not have an API defined for them, there is some API for them in DIF DID Registration (which #379 refers to); but the recover operation is not standard except in Sidetree.