Support `$dynamicAnchor` and `$dynamicRef`

### Motivation

These keywords are defined in the JSON Schema 2020-12 dialect.

Release notes: https://json-schema.org/draft/2020-12/release-notes

These are useful for representing generics: https://json-schema.org/blog/posts/dynamicref-and-generics

Specification of dynamic scope rules: https://json-schema.org/draft/2020-12/json-schema-core#name-lexical-scope-and-dynamic-s

### Proposed solution

Schemas containing `$defs` generate types that include the defs as members. If the schemas contain `$dynamicAnchor`, they create protocols with associated types instead of concrete types with type aliases. The schema of the dynamic-anchored types are used as the type constraint for the associated types.

Dynamic reference schemas (those containing the `$dynamicRef` keyword) refer to associated types and may supply generic parameters using their own `$defs` and `$dynamicAnchor` keywords.

### Alternatives considered

Avoid generating protocols / associated types, and instead always generate concrete types, treating dynamic refs/anchors as template instantiation.

### Additional information

This would depend on support in OpenAPIKit. Upstream issue filed: https://github.com/mattpolzin/OpenAPIKit/issues/359

Start by reviewing the JSON Schema 2020-12 dynamic scope specification and the linked OpenAPIKit issue #359, since the proposed implementation depends on upstream support. Done means schemas using $dynamicAnchor and $dynamicRef generate the described protocols, associated types, and dynamic references in Swift.

Written by the indexing model from the issue text.