EcdsaSecp256k1Signature2019's getVerificationMethod should support DID Dereferencing (while keeping backwards compatibility with current implementation)

[JaceHensley]

See digitalbazaar/jsonld-signatures#141 for more discussion.

Ultimate Problem

documentLoaders should be returning a dereferenced DID Document not just resolving to the DID Document itself. The getVerificationMethod needs to be updated to support deferencing of DIDs instead of just resolving. In a future version support for resolved DID Document will be dropped