Add @context, alsoKnownAs, controller to DIDDoc

[tdiesler]

• https://www.w3.org/TR/did-core/#also-known-as
• https://www.w3.org/TR/did-core/#did-controller
• https://www.w3.org/TR/did-core/#json-ld

[victormartinez-work]

@dkulic @yvgny Can we check if Thomas recommendations are included in other didcomm libraries, specially didcomm python? Let's duplicate the issue in the other libraries if needed

[tdiesler]

@context is critical because it seems to be an required attribute and WaltId (correctly) rejects docs that don't have a @context

The JSON-LD Context is either a string or a list containing any combination of strings and/or ordered maps.

[tdiesler]

PR: #77