Advanced search should be configured in REST instead of Angular

[ybnd]

Describe the bug
The new advances search sidebar section in the Angular UI introduced in DSpace/dspace-angular#2608 depends on

• Angular-side configuration for the available filters
• Operator values hardcoded within an Angular template

This will make customizing discovery configurations more error-prone, as the advanced filters configured in Angular may not correspond to existing filters. Furthermore, this setup makes it impossible to vary whether/which advanced filters are shown in different search contexts.

Our proposed solution is to move this configuration to discovery.xml in the backend:

• A new property can be added to DiscoveryConfiguration, representing the list of filters made available for advanced search
  • Leaving this list unset or empty would turn off advanced search for this configuration
  • This list of filters can be taken into account when indexing (in case a specific filter is only available in advanced search)
• We can configure a subset of this list, as is the case now (but hardcoded in Angular)
  • Currently, all filters share the default list of operators. If this is expected to change, we could also configure a mapping of filters to operators
• This information can be exposed via the REST API in SearchConfigurationRest
• The frontend implementation of the advanced filter section can then be refactored to make use of this configuration

Expected behavior
Discovery configuration that interacts with specific filters should remain close together for ease of customization

Related work
Related to #8431 and DSpace/dspace-angular#2608