sql: Implement `count(distinct ..)` for `daft-sql` crate

[raunakab]

Is your feature request related to a problem?

AggExpr::CountDistinct(..) is not implemented inside of daft-sql/src/modules/agg.rs.

Describe the solution you'd like

AggExpr::CountDistinct(..) should be handled by to_expr inside of daft-sql/src/modules/agg.rs.

Describe alternatives you've considered

No response

Additional Context

No response

Would you like to implement a fix?

Yes