Queries and projections

[jtheisen]

We need a way to load extents based on queries, ie.

    public static async Task<E[]> LoadExtentsAsync<E>(this DbContext db, IQueryable<E> query, Action<ExtentBuilder<E>> extent)
        where E : class
    {
        ...
    }

One of the tricky parts is that really we also need this to work when E isn't an entity (called a projection because the model is projected onto a class that's not part of the model). Projections are quite common.

This likely necessitates further builder related extensions as we probably need to tell Reconciler that E isn't an entity, ie. I'm not sure we can make the following syntax work:

db.LoadExtentsAsync(db.Stuff.Select(s => new { Stuff = s, More = ... }), e => e.WithOne(s => s.Stuff))

(we may have to use a method distinct from WithOne here)