Modify context implementation in did-utils to also contain objects

[francis-pouatcha]

The current context object defined in did-utils/src/ldmodel.rs is not complete.

[francis-pouatcha]

Example of context containing objects:

{
  "@context": [
      {"title": "https://schema.org/title"},
      "https://w3id.org/security/data-integrity/v1"
    ]
  }

Context can also have this form:

{
"@context": {"title": "https://schema.org/title"},
}

or this form:

{
  "@context": [
    {"title": "https://schema.org/title"},
    "https://w3id.org/security/data-integrity/v1"
  ]
}

[francis-pouatcha]

Context can also be a single string:

{
  "@context": "https://w3id.org/security/data-integrity/v1"
}

[francis-pouatcha]

This form is already supported in the current impl.

{
"@context": [
    "https://www.w3.org/ns/did/v1",
    "https://w3id.org/security/data-integrity/v1"
  ]
}

[francis-pouatcha]

Also

{
  "@context": [
    {"title": "https://schema.org/title", "title2": "https://schema.org/title2"},
    "https://w3id.org/security/data-integrity/v1"
  ]
}