[ENH] Support UNIX_$timeUnit functions

[randerzander]

Related to #412

I'm trying to do math on timestamps, but ran into #411 and #412.

The Calcite docs suggest

UNIX_SECONDS(timestamp) | Returns the number of seconds since 1970-01-01 00:00:00

But that fails to parse:

import pandas as pd
from dask_sql import Context

c = Context()

df = pd.DataFrame({'dt0': ['2022-03-01 12:00:00'], 'dt1': ['2022-03-01 13:00:00']})
df['dt0'] = df['dt0'].astype('datetime64[s]')
df['dt1'] = df['dt1'].astype('datetime64[s]')

c.create_table('test_dt', df)

c.sql("SELECT UNIX_SECONDS(dt0) FROM test_dt").compute()

---------------------------------------------------------------------------
ParsingException                          Traceback (most recent call last)
Input In [123], in <module>
----> 1 c.sql("select UNIX_SECONDS(dt0) FROM test_dt").compute()

File ~/conda/envs/dsql-2-28/lib/python3.9/site-packages/dask_sql/context.py:458, in Context.sql(self, sql, return_futures, dataframes, gpu)
    455     for df_name, df in dataframes.items():
    456         self.create_table(df_name, df, gpu=gpu)
--> 458 rel, select_names, _ = self._get_ral(sql)
    460 dc = RelConverter.convert(rel, context=self)
    462 if dc is None:

File ~/conda/envs/dsql-2-28/lib/python3.9/site-packages/dask_sql/context.py:892, in Context._get_ral(self, sql)
    887     logger.debug(f"Original exception raised by Java:\n {e}")
    888     # We do not want to re-raise an exception here
    889     # as this would print the full java stack trace
    890     # if debug is not set.
    891     # Instead, we raise a nice exception
--> 892     raise ParsingException(sql, str(e.message())) from None
    894 # Internal, temporary results of calcite are sometimes
    895 # named EXPR$N (with N a number), which is not very helpful
    896 # to the user. We replace these cases therefore with
   (...)
    899 # but so far I did not find such a case.
    900 # So please raise an issue if you have found one!
    901 if sqlNodeClass == "org.apache.calcite.sql.SqlOrderBy":

ParsingException: Can not parse the given SQL: From line 1, column 8 to line 1, column 24: No match found for function signature UNIX_SECONDS(<TIMESTAMP>)

The problem is probably somewhere here:

	select UNIX_SECONDS(dt0) FROM test_dt
	       ^^^^^^^^^^^^^^^^^

cc @ayushdg , @jdye64