Autogenerate OpenAPI spec for InvenioRDM instances based on config

Maintaining an OpenAPI spec for the InvenioRDM REST API by manually editing YAML files has certain caveats:

- Depending on the instance configuration, some of the documented fields and accepted values for payloads might not be valid input
- E.g. when `RDM_ALLOW_METADATA_ONLY_RECORDS = False` is set, one cannot provide `files.enabled: false` when updating a draft.
- Editing YAML files is (ironically) not very human-friendly
- As new endpoints and features are added, the spec can fall out of sync without a clear way on how to tie it to specific InvenioRDM versions.

Given these issues, we can consider a solution for autogenerating the OpenAPI spec using code. This would be an Invenio module that:

- Generates, caches (?), and serves the OpenAPI spec under an endpoint (e.g. [`/.well-known/api-catalog`](https://www.rfc-editor.org/rfc/rfc9727.html), `/api/spec.json`, ...)
- Takes into account instance configuration when generating the different endpoint specs and examples
- Allows for modular definitions of endpoints and customizing order/grouping, naming, auth methods, etc.

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### Rejected approaches

- Libraries that autogenerate OpenAPI specs from Marshmallow schemas (e.g. [`apispec`](https://github.com/marshmallow-code/apispec)), will not work well for our cases becuase:
- We make heavy customized use of Marshmallow with `@{pre,post}_{load,dump}` and custom types
- Often, the output of these libraries is not enough, and one has to post-process it anyway to fix the output.
- Usage of these tools has to be integrated into existing code, which would increase the already complex (de)serialization logic we have.