expose connectorx 'protocol' param to "read_sql" so it can work with redshift

[alexander-beedie]

Related to (but not dependent on) sfu-db/connector-x/#187. This is a small patch that adds the 'protocol' param to polars' read_sql so that it can be passed-through to connectorx; this is required for redshift backends as the param has to be set to a non-default value ('cursor' instead of 'binary'), otherwise loading fails. No change to default behaviour or other parameters.

BEFORE

>>> # fails to execute against redshift dbs
>>> df = pl.read_sql( connection_uri='postgresql://...redshiftdb...', sql='SELECT * FROM table' )
RuntimeError: db error: ERROR: syntax error at or near "("

AFTER

>>> # successfully loads redshift data into polars frame
>>> df = pl.read_sql( connection_uri='postgresql://...redshiftdb...', sql='SELECT * FROM table', protocol='cursor' )

Also added some additional test coverage for read_sql, and made some trivial docstring updates (in line with the connectorx patch referenced above). Thanks!

[alexander-beedie]

(apologies for the slightly noisy commit; happy to squash it or rebase/recommit if you prefer ;)

[ritchie46]

(apologies for the slightly noisy commit; happy to squash it or rebase/recommit if you prefer ;)

No worries, I'll squash! Thanks for the PR. :)