feat: Add support for COUNT(DISTINCT expr)

[andygrove]

Which issue does this PR close?

Part of #1267

Rationale for this change

Increase coverage of TPC-H benchmark.

What changes are included in this PR?

Add support for COUNT(DISTINCT expr)

How are these changes tested?

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.40%. Comparing base (f09f8af) to head (d11342e).
⚠️ Report is 435 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2273      +/-   ##
============================================
+ Coverage     56.12%   57.40%   +1.27%     
- Complexity      976     1287     +311     
============================================
  Files           119      145      +26     
  Lines         11743    13370    +1627     
  Branches       2251     2374     +123     
============================================
+ Hits           6591     7675    +1084     
- Misses         4012     4429     +417     
- Partials       1140     1266     +126     

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[comphead]

That looks incredibly useful!