Design of MaterializedViewQuery

Currently, #5556 implements it as `MaterializedViewQuery` and `MaterializedViewQueryQueryToolChest`, i. e. in the same type framework as `Query` and `QueryToolChest`. However, `MaterializedViewQuery` and `MaterializedViewQueryQueryToolChest` are shallow and pass-through classes. `MaterializedViewQueryQueryToolChest` couldn't sensibly return something from `getResultTypeReference()`, so I wonder why it didn't throw NPE in `DirectDruidClient` before, but I hit this problem in my change #6898.

There should probably be an abstraction like "query modifier" and other parts of the infrastructure should recognize it.

cc @zhangxinyu1 @jihoonson

Start with MaterializedViewQuery and MaterializedViewQueryQueryToolChest from #5556, then trace how DirectDruidClient uses getResultTypeReference(). Review the interaction with change #6898 and the existing Query and QueryToolChest abstractions. Done means the query-modifier design is defined and the affected infrastructure handles materialized-view queries without requiring an inappropriate result type.

Written by the indexing model from the issue text.