Allow search on resources outside the project scope

This is an extension of https://github.com/BlueBrain/nexus/issues/2737 where clients can search out of the scope of a project.

## Endpoints
- POST `v1/resources/_search?viewId={viewId}`: Search on all accessible projects for at the view(s) whose ids are passed as a query parameter.
- POST `v1/resources/sparql?viewId={viewId}`: Search on all accessible projects for at the view(s) whose ids are passed as a query parameter
- GET `v1/resources/sparql?query={query}&viewId={viewId}`: Search on all accessible projects for at the view(s) whose ids are passed as a query parameter

Notice that what's currently implemented in the search plugin is just a concrete use case for this more generic endpoint, the one under: `v1/resources/_search?viewId=searchView`

Start by tracing the three listed resource search endpoints and comparing them with the existing search-plugin use case at v1/resources/_search?viewId=searchView. Done means each endpoint can search all accessible projects using the view IDs supplied in viewId, while preserving the stated access boundary.

Written by the indexing model from the issue text.