External (schema.org) type definitions in Guardian schemas

[anvabr]

Problem description

Schema.org have published many schemas and standards which defined common data types used in various industries. Supporting these in Guardian would help with standardising the basic vocabulary used in schemas, and help users by validating field values and imported schemas automatically.

Requirements

• Ensure Guardian schemas can reference schema types defined/published on shema.org such that schema/fields/values get validated on every save/load/update operations.
• Policy creators should be able to use any future new datatypes published on schema.org in their Guardian policies without changes to Guardian (no upgrades for this purpose should be required)

Definition of done

The usage of schema.org type is validated, documented and demonstrated at least in one of the Guardian sample policies.

Acceptance criteria

Using schema.org datatype does not require any additional or special effort or knowledge from Guardian policy authors.

[anvabr]

Potentially consider changing the location of context field in VC documents, which is currently inside the credentialSubject section but it probably needs to be at the top level of the structure of the VC document itself.