[Feature] Composite Count Distinct should be identified by the IR

[Selvomega]

User need and MVP

The COUNT(DISTINCT blah) where blah is a composite of columns should be supported

Authoritative design

Do not understand

Inputs, outputs, and invariants

Not applicable. Or the input should be a SQL query with composite distinct-count, output is such query safely get parsed and passes all components of the planner (although may not be optimized)

Before and after behavior

before the SQL query with composite distinct-count cannot be supported
after if can

Unit and end-to-end acceptance tests

Unit test is just to test if the unsupported SQL with composite count-distinct can be supported

Explicit non-goals

Not applicable