[ENH] Add support for CONCAT function

[ayushdg]

Is your feature request related to a problem? Please describe.
Dask-sql supports concatenating columns via the || operator but does not support the CONCAT method.

Describe the solution you'd like
Supporting CONCAT as follows:
SELECT CONCAT('John', ', ', 'Doe')

Describe alternatives you've considered
Workaround to this is using the || operator
SELECT 'John' || ', ' || 'Doe'

Additional context
While checking this feature I also came across CONCAT_WS which is a part of the postgresql featureset but this doesn't get parsed by calcite.

[charlesbluca]

Seems like all we need to do here is add concat to the operation mapping for RexCallPlugin - opened #253 to do this