Make `pyodbc` in the spark package an optional add on

[JCZuurmond]

pyodbc is required for the Spark dialect, see here. We can make this an optional add-on, which would by install like:

python -m pip install "soda-sql-spark[ODBC]"

Two things need to be done:

• use extras_requires to install pyodbc with key ODBC
• only import pyodbc when the odbc connection method is used

This would make the installation lighter, which is especially useful for odbc since it requires unixodbc to be installed which is not always the case.