[ENH] coalesce implementation

[mattf]

Is your feature request related to a problem? Please describe.
queries using coalesce cannot be run by dask-sql

for instance,

import dask_sql as ds
import pandas as pd

print(ds.__version__)

c = ds.Context()
products = {'product_id': [1, 2], 'wholesale_price': [1.00, 2.00]}
sales = {'product_id': [1]*2 + [2]*5, 'retail_price': [2.00]*2 + [5.00]*5, 'quantity': [1, 2, 4, 8, 16, 32, 64], 'city': ['SF', 'SJ', 'SF', 'SJ', 'Maimi', 'Orlando', 'SJ'], 'state': ['CA']*4 + ['FL']* 2 + ['PR']}
c.create_table('products', pd.DataFrame(products))
c.create_table('sales', pd.DataFrame(sales))

c.sql("SELECT COALESCE(NULL, city) FROM sales")

results in,

2022.10.1
...
NotImplementedError: coalesce not (yet) implemented

[mattf]

addressed by #823 in 2022.12.0