External (schema.org) type definitions in Guardian schemas

### Problem description

[Schema.org](http://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 [schema.org](http://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](http://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](http://schema.org) type is validated, documented and demonstrated at least in one of the Guardian sample policies.

### Acceptance criteria

Using [schema.org](http://schema.org) datatype does not require any additional or special effort or knowledge from Guardian policy authors.

Start by locating Guardian's schema validation path and the sample policies, then determine how external schema.org types are resolved during save, load, and update operations. Done means schema.org types require no special policy-author knowledge, remain usable as new types are published, and are validated, documented, and demonstrated in a sample policy.

Written by the indexing model from the issue text.