Add support for CONCAT

[charlesbluca]

Adds concat to the RexCallPlugin with the same operation as ||, allowing for either style of string concatenation.

[codecov-commenter]

Codecov Report

Merging #253 (b2bc641) into main (767a23c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #253   +/-   ##
=======================================
  Coverage   99.61%   99.61%           
=======================================
  Files          64       64           
  Lines        2608     2608           
  Branches      367      367           
=======================================
  Hits         2598     2598           
  Misses          8        8           
  Partials        2        2           

Impacted Files	Coverage Δ
dask_sql/physical/rex/core/call.py	100.00% <ø> (ø)

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[quasiben]

Thanks @charlesbluca