Fixed support for using multiple DbContexts

[bart-degreed]

Fixes #739.

Added example project with tests, as this is tricky to get right (and not accidentally break in the future).

In the past, this was accomplished by calling ResourceGraphBuilder.AddDbContext() from services.AddJsonApi(). I've changed that to instead accept a collection of DbContext types in the non-generic overload of services.AddJsonApi(), so that we can also register matching resolvers.

This also fixes InverseNavigation to loop over all DbContexts instead of a single one.

And for this to work, I found another bug where ResourceService<TResource> was depending on injected IResourceRepository<TResource,TId> instead of IResourceRepository<TResource>, so it would pick the wrong repository.

Finally, I simplified caching of scanned assemblies that was spread over multiple layers doing duplicate work.

[maurei]

I am aware this PR only fixes support for multiple DbContexts rather than introducing it as a new feature or improving it as a feature. But regardless, perhaps we can consider improving the follow.

I think it is unnecessarily verbose having to both pass along DbContextA, DbContextB to AddJsonApi() call and also having to register a IDbContextResolver implementation for each.

We could register a generic resolver service DbContextResolver<TDbContext> like this

public class DbContextResolver<TDbContext> : IDbContextResolver where TDbContext : DbContext
{
    private readonly DbContext _context;

    public DbContextAResolver(TDbContext context)
    {
        _context = context;
    }

    public DbContext GetContext()
    {
        return _context;
    }
}

Then a dev can just inject DbContextResolver<DbContextA> in his custom repository.

Alternatively, if we do not want to offer this solution out-of-the-box but want to leave the responsibility with the developer, I would propose to change the example using this rather approach than two separate DbContextResolver services. It is cleaner and as such easier to follow.

Other than this I have nothing to comment on the PR. Looking good 👍

[bart-degreed]

I am aware this PR only fixes support for multiple DbContexts rather than introducing it as a new feature or improving it as a feature. But regardless, perhaps we can consider improving the follow.

Great suggestion! I've pushed another commit to address this.