refactor(30463): Fix bugs in the OpenAPI specs of the data combining feature

[vanch3d]

See https://hivemq.kanbanize.com/ctrl_board/57/cards/30463/details/

Implementing the frontend part of the data combining has highlighted a few mis-specifications of the API that need correction. Most are cosmetic changes (reorganised the structure of schemas to facilitate the visualisation of the editor), but a few are breaking changes:

• a schema property is added to the destination of DataCombining
• a DataIdentifier schema is being created to provide a "namespaced" reference to both tags and `topic filters
• two new API endpoints are added
  • GET /api/v1/management/topic-filters/{filter}
  • GET /api/v1/management/topic-filters/{filter}/schema

Out-of-scope

• Other issues might be worth fixing at some point but are not required (e.f. the use of items: {} in schemas). They will be dealt with in another ticket

[codepitbull]

Just one wording change for id vs name.

[vanch3d]

Incorrect specifications of the routes; need fix

[DC2-DanielKrueger]

We have two open api specs, this one and the 2025.4-SNAPSHOT one, we must decide on which one to use

[codepitbull]

One comment about XML naming conventions.