[FEATURE] Support customizing the generated OpenAPI JSON schema

After a discussion on softwaremill.community (https://softwaremill.community/t/differentiating-between-undefined-and-null/154/8), @adamw suggested opening an issue.

In short, this is a feature request for some way to customize what the generated OpenAPI JSON schema will be for a type.

In my particular use case, I needed the option to set "nullable" to true for a specific type, not for all optional types (that is already supported).

It is possible to manually, eg. with quicklens, modify the OpenAPI returned from OpenAPIDocsInterpreter(). toOpenAPI(...), but it's not very elegant.

Adam's suggestion from the discussion:

As for a more user-friendly fix, I think we should have an option to customise the final, generated JSON Schema (which is part of the OpenAPI object). We could set an attribute on the tapir-schema, with a function which would be called by the OpenAPI interpreter.

Start with OpenAPIDocsInterpreter().toOpenAPI(...) and the tapir-schema customization point described in the issue. Review how the interpreter produces the JSON Schema within the OpenAPI object and determine the API needed for per-type customization. Done means a specific type can customize its generated schema, including nullable=true, without changing all optional types.