Implement `did:ethr` resolver

[Patrik-Stas]

• Demo of JS implementation did:ethr in https://github.com/gmulhearn/anoncreds-on-ethereum/pull/4 can be a good place to get the feet off the ground
• Prior to this work, we should stabilize DidDocument - remove the current use of generics - and all the changes which trickle up as result

[gmulhearn]

Will give this one a go