DIDMethodResolver

[amika-sq]

My previous attempt at adding a DIDMethodResolver protocol in #9 didn't really work out how I intended it to, so I reverted it.

This PR re-introduces a DIDMethodResolver protocol, and handles the behaviors of a DID resolver much better.

DIDMethodResolver

DIDMethodResolver is a new protocol that defines what's required to resolve a DID method. There are two things:

1. the methodName that's being resolved
2. a func resolve(didURI: String) -> ResolutionResult function

Each DID method implements this DIDMethodResolver with a struct. This struct can then initiated and registered with DIDResolver, to customize the resolution experience for any SDK consumer's use case.

DIDResolver

DIDResolver now has a new function available on it:
public static func register(resolver: DIDMethodResolver)

This is how an SDK consumer can customize the behavior of the resolution of any DID Method. For example, if a user wanted to customize the experience of resolving did:ion, they would do the following:

let resolver = DIDIon.Resolver(options: .init(gatewayURI: "https://my-custom-gateway.org"))
DIDResolver.register(resolver: resolver)