Mixed type properties for DIDs and VCs

[KendallWeihe]

This ticket will take the place of the following

• didcore.Document @context non-string types #92
• Add mixed types to Document Controller #101
• Add mixed type to Document Service Endpoints #102
• Add mixed types for embeddable Verification Methods #103

Also relevant as a source of inspiration for this work is this PR #104

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The source of truth as to what web5 will support is codified in the web5-spec repo, and initially created within this PR.

The requirements are as follows:

• Implement support for the types defined in the web5-spec repo (at the time of this ticket, that'll be, DIDs and VCs)
• During deserialization of a type, given the string/byte-array/whatever which is being deserialized has a property type which is different than what we have defined in the web5-spec repo, then throw an exception
  • Consider if JSON Schemas can be used prior-to or during deserialization as a mechanism of implementing this expected behavior (web5-spec issue for tracking) -- rather than writing custom JSON unmarshallers, perhaps we could validate the type with a single JSON Schema validation call

Note: the only mixed type I see at this time is the issuer in the VC Data Model. As for how we will implement a mixed type here in golang, we do not have a set consensus, but have two separate proposals:

1. Rely on the caller to perform type assertions; From this PR notice the dids/didcore/document_test.go or dids/didcore/examples_test.go as a reference example
2. @alecthomas made a separate proposal which is self-evident in his comment here

This work may prove too unwieldy for a single PR, if so then piece meal PRs are appropriate

[KendallWeihe]

We should use the JSON schema to enforce existence requirements, so no need to write custom json marshallers/unmarshallers in this work. Wrote a new ticket for JSON Schema validation #131