Support custom queries

[wvengen]

Currently, only SQL queries are supported. It would be great if native queries would also be supported.

[wvengen]

With the current approach of altering the current query, this would mean that the query type would need to be changed (since the materialized view can only be referenced using SQL - or can we reference it from a custom query? that may be a good alternative).

[wvengen]

For SQL queries one can still edit the existing query, and press refresh.
For custom queries, if the query is modified to reference the materialized view, it cannot be edited in Metabase anymore (one would need to unmaterialize the question, edit it, and materialize it again).

It may be necessary for UX to change the approach, and keep the original query, creating a new question from the materialized view. Or, create a new question as a copy of the original, and modify the original question to reference the materalized view. The latter option makes it easier reference the query other queries, and not having to update these when switching between live and materialized.