Allow to use object as serviceEndpoint in did dht

[jakubkoci]

According to the did-core spec:

The value of the serviceEndpoint property MUST be a string, a map, or a set composed of one or more strings and/or maps.

That would enable using the did:dht with the DIDComm protocol as it is defined at DIDComm protocol spec.

I already created a PR #990 demonstrating a way how to implement it.