Add support for DISTINCT projections in `decorrelate_where_exists`

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

Setup

$ export DATAFUSION_OPTIMIZER_SKIP_FAILED_RULES=false
$ echo "1,2" > test.csv
$ cd datafusion-cli
$ cargo run

❯ create external table test (a int, b int) stored as csv location 'test.csv';

Test

This query works:

❯ select * from test where exists (select a from test t2 where test.a = t2.a);
+---+---+
| a | b |
+---+---+
| 1 | 2 |
+---+---+

If I add the DISTINCT keyword then the optimizer fails:

❯ select * from test where exists (select distinct a from test t2 where test.a = t2.a);
Internal("Optimizer rule 'decorrelate_where_exists' failed due to unexpected error: cannot optimize non-correlated subquery at /home/andy/git/apache/arrow-datafusion/datafusion/optimizer/src/decorrelate_where_exists.rs:141\ncaused by\nError during planning: Could not coerce into Filter! at /home/andy/git/apache/arrow-datafusion/datafusion/expr/src/logical_plan/plan.rs:1157")

Describe the solution you'd like
Support distinct projections in subqueries

Describe alternatives you've considered
None

Additional context
None