Implement basic COALESCE functionality

[ChrisJar]

[randerzander]

Will depend on https://github.com/dask/dask/pull/9563 and related dask-cudf work

[codecov-commenter]

Codecov Report

Merging #823 (22cf6ad) into main (d2896fa) will increase coverage by 0.26%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #823      +/-   ##
==========================================
+ Coverage   75.67%   75.94%   +0.26%     
==========================================
  Files          73       73              
  Lines        4050     4065      +15     
  Branches      731      737       +6     
==========================================
+ Hits         3065     3087      +22     
+ Misses        824      814      -10     
- Partials      161      164       +3     

Impacted Files	Coverage Δ
dask_sql/physical/rel/logical/aggregate.py	90.10% <100.00%> (+0.10%)	:arrow_up:
dask_sql/physical/rex/core/call.py	81.50% <100.00%> (+0.47%)	:arrow_up:
dask_sql/_version.py	35.31% <0.00%> (+1.41%)	:arrow_up:
dask_sql/mappings.py	89.21% <0.00%> (+1.96%)	:arrow_up:

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[ayushdg]

@ChrisJar are the changes in https://github.com/dask/dask/pull/9563 only needed to enable this functionality on gpu's or is this needed for this functionality in general?

[ChrisJar]

Those changes are actually not needed for this PR at all. I ended up taking a different path.