Refactor Backend Architecture for the remaining Repositories, AASXFileServer and AasDiscoveryService

[mateusmolina-iese]

Linked to the 1st Phase of #584

This PR refactors the backend architecture for the Submodel, AAS, ConceptDescription repositories/services, the AASXFileServer, and the AasDiscoveryService to (1) rely on existing CrudRepository implementations when available -- for example, MongoDB -- and (2) use backend-specific implementation for the services.

Following a similar implementation strategy of #570, with the following changes:

1. The AAS/SubmodelOperations backend-specific fragment is now registered by adding the corresponding implementation bean to the META-INF/spring.factories file, instead of relying on a BeanPostProcessor.
2. The Aas/SubmodelServiceFactory is reused in the AAS/SubmodelRepository. This change was necessary so that the repositories could reuse the features implemented at the service level. To work without compromising the added benefits of the refactoring, a similar architecture to the one used in Aas/SubmodelRepositories was implemented in the Aas/SubmodelServices based on the CrudRepository (from Spring).

Other than that, this PR brings several other changes:

1. A more consolidated and error-proof mapping of a given entity with the collection name for the MongoDB backends
2. An UUID-based generation of package IDs for the AASXFileServer -- enabling the persistence of the packages stored
3. Refactor AASXFileServer to store packages using the unified FileRepository
4. Add support to MongoDB backend for the AASService
5. General clean-code-related fixes in tests and removal of obsolete/unused code related to the backend

Suggestions for future work:

1. Remove the service-related methods from the interfaces of the AAS/SubmodelRepository, eliminating their dependency on the AAS/SubmodelServiceFactories. This greatly simplifies the dependency structure and architecture and avoids unnecessary proxies.
2. Instead, the AAS/SubmodelServiceFactories should be used directly in the AAS/SubmodelControlers as a component for implementing the service-related methods.