Add did resolver interface

[TimoGlastra]

As part of the refactoring and making the framework more generic I think it would be good to adopt the did-resolver library. This allows for pluggable did methods, making use of the already available did methods.

https://github.com/decentralized-identity/did-resolver

Another thing that this will bring is support for the did:web did method and the .well-known did configuration. I think this would be great for mediators (we now use a custom format).

agent.dids.resolve("did:xxx")

class DidResolverService {
	public resolve(): Promise<Did>
}