Ref T8441. Ref T7715. Add a layer of indirection to fields in search engines. This will allow us to:
- Simplify SearchEngine code, which has collected a lot of duplication around expressing what is effectively field types.
- Automatically add fields like "Spaces" and "Projects" (primary driver for T8441).
- Reorder or hide fields (not sure if we really want to do this, but it seems plausible, and this will let us play around with it, at least).
- Drive Conduit Query methods via SearchEngines, so the same code specifies both the search UI and the application.query endpoint (primary driver in T7715).