PortableDID refactoring part 1

[amika-sq]

First part in the changes to DID creation, PortableDID, and BearerDID from here.

This PR addressed the following refactoring areas:

• BearerDID now contains the following properties:
  • uri - the DID URI represented
  • document - the full DIDDocument for the BearerDID
  • keyManager - the KeyManager where the backing private keys are stored
  • metadata - a grab bag dictionary/map that can store method-specific information
• BearerDID can now be exported via the export() function, which returns a PortableDID
• PortableDID can be imported via a DID Method
  • DIDJWK is the only method currently implemented that can import
  • import() takes in an exported PortableDID, and an optional KeyManager to store the private keys into

I will address the following in a follow-up PR:

• BearerDID needs to expose a getSigner function