[bug] DidDocument type for serviceEndpoint is too strict

[gmulhearn]

According to the spec, serviceEndpoint may be a URL string, or a map or set of URLs and/or maps. https://www.w3.org/TR/did-core/#dfn-serviceendpoint

however the current vcx impl has it hardcoded as service_endpoint: Url. We should change it to be an enum of different types.

After this is complete, we may need to also update our did:peer resolvers, as they are known to use this structure in examples for services: https://identity.foundation/peer-did-method-spec/#creating-a-did