services: provide base class for DB-only based entities

[slint]

• We currently have a base RecordService class that assumes "Record-like" models backed by both the DB as a primary store for item read/write operations and OpenSearch indices for all list/search operations.
• When creating new entities (i.e. the model + service + resource trifecta) that are only stored in the DB and are not backed by OpenSearch indices, we usually subclass from RecordService and override/adapt/remove configuration and functionality that assumes any OpenSearch indexing to get things working. This is necessary because the base class has a lot of convenient functionality for schema validation, results rendering, serialization, and overall ease of configuration.

We have the following use cases so far of services that are DB-only and had to do this:

• OAIPMHServerService
• Invenio-Jobs
• VocabularyTypeService

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The proposal is to "lift" some of the functionality overridden/reimplemented in the above use cases into invenio-records-resources, to avoid duplication and benefit across the board from common utilities and features. Some of this common functionality includes:

• Service result classes that can handle DB-paginated objects
• DB-based sorting/filtering/pagination on the Service.search() method
• Specialized Unit of Work operations (instead of custom-defined)
• Removal/skipping of any OpenSearch-specific functionality (services: skip index rebuilding invenio-jobs#42)
• (Optional) possibly aggregations/facets?