Update DataGateway API OpenAPI/Swagger Docs to ICAT Backend

[MRichards99]

Description:
For DataGateway API, we use the OpenAPI specification to document the API. This implementation is in itself slightly tailored to the database backend, but for the most part explains the API well.

The database and ICAT backends have differences, mainly with the field names. Since the database backend isn't being used in production, most users will use the ICAT backend due to the additional features it may bring.

The OpenAPI docs should be updated for the ICAT backend, using Python ICAT to generate the fields etc. Ideally, the OpenAPI interface for DataGateway API should be generated depending on the backend being used. This way, the existing code won't go to waste and all the use cases will be covered (i.e. the docs will be accurate for both backends).

I'm not sure how many people use the Swagger interface, so I don't think this is particularly high priority.

Acceptance criteria:

• Check that the existing OpenAPI docs match DataGateway API's Python ICAT backend. Make changes as needed, this backend should be prioritised over the database backend
• If possible, determine which backend is being used and use the appropriate code/docs (i.e. existing code/docs for DB backend, new docs for ICAT backend)