Add support for specifying JSON schema dialects

[mattpolzin]

See Specifying Schema Dialects in the new spec.

This change will impact both the JSONSchema type and the root document type (where a default dialect is allowed to be specified).

Note that OpenAPIKit will not (at this time) do anything with values of these fields other than store/encode/decode them. In the future we may indicate what values (i.e. what schema dialects) are supported by OpenAPIKit but for now we will not change the behavior of OpenAPIKit in response to the dialect specified.

Work on types in the OpenAPIKit (not OpenAPIKit30) module.