build in helpers to resolve URIs against base URIs

[mattpolzin]

There is logic worth coding into OpenAPIKit so downstream can just ask the relevant questions instead of doing the logic themselves.

See https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.2.0.md#establishing-the-base-uri and https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.2.0.md#appendix-f-examples-of-base-uri-determination-and-reference-resolution for details on what should be offered and note that there can be $id and $ref all over the place and $self on the root document.