bug(typing): Internal type exposed on public interface

[peterschutt]

We expose SchemaCreator on DTOFactory.create_openapi_schema()`, but its an undocumented type.

We should replace it with a protocol.

          This weird wording is because `SchemaFactory` is an internal type, but its exposed on a public interface here. We should probably replace the annotation with a Protocol.

Originally posted by @peterschutt in #3371 (comment)

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