New config option in mixin to use native histograms only in dashboards

[krajorama]

Currently Mimir dashboards execute queries on some latency histograms using both native and classic histograms. This puts extra load on the backend and can be slower than just running one of the queries.

Add a mixin configuration parameter to be able to restrict which kind of query is run at build time.
MVP: allow generating dashboards with either both queries or only native histograms queries.

This would not be necessary if grafana/grafana#79848 was implemented or PromQL would optimize away impossible queries (x < -Inf).