Add the entity list resource base class with both query sources

Problem/Motivation

Listings are the second most common kind and the one with the most shared behaviour to get right once: pagination, sorting, filtering, the redirect a page past the end produces, the breadcrumbs, and the facets when a search index backs the listing. The forks implement all of this per resource, which is where the pagination bugs live.

Proposed resolution

• MyrestEntityListResourceBase serving both sources: an entity query, and a search_api index when the convention names one.
• Pagination, sorting and filtering handled once, with the parameter names the module already documents in its OpenAPI.
• The out-of-range redirect response kept exactly as it is documented today.
• Facets delegated to the facets convention; absent convention, absent key.
• Cache contexts that actually reflect the query parameters — the current per-resource implementations disagree with each other about this.

Remaining tasks

Everything.