Support resolution of "key" DID method

[edchapman88]

New modules:

• trustchain-ion/did_methods
  • New struct DIDMethodsResult.
    • In short, this struct can be used in place of a resolver to handle the resolution of multiple DID methods.
    • Contains a map of DIDMethods and implements the DIDResolver trait to expose the resolve method with a prior match on the method of the DID being resolved (eg. "ion" or "key" methods).
    • DIDMethodsResult is a wrapper for ssi::did::DIDMethods, re-implementing the full DIDMethods api in addition to a resolve_as_result method, which is a Trustchain extension of the resolve method (wrapping the returned tuple in a Result<T,ResolverError>)

Extent of DIDMethodResult usage:

• Currently only used in resolution integration tests found in trustchain-ion/tests/resolver.rs.
• IONResolvers are used extensively in tests and library code, created with get_ion_resolver, and none of these have been replaced with the more flexible DID Methods resolver (in many cases it doesn't make sense to support resolution of non-"ion" DIDs, eg. when resolving during ion verification inside the IONVerifier).

Changes with widespread effects:

• get_ion_resolver(endpoint: &str) refactored to get_ion_resolver(endpoint: &Endpoint), using the Endpoint type from the trustchain-ion