RUBY-723 Support the $meta projection operator and sort field

The driver should allow projection operator values to be hashes to permit the following code:

    coll.find_one({ :$text => { :$search => 'spam' }}, 
                  { :fields => [:t, { :score => { :$meta => 'textScore' } }] })


Similarly, sort values can also be hashes:

    assert coll.find({ :$text => { :$search => 'spam' }}).sort(:score => { '$meta' =>'textScore' })