implement icontains, istartswith, iendswith operators

implement icontains, istartswith, iendswith operators

Added long-requested case-insensitive string operators
:meth:`_sql.ColumnOperators.icontains`,
:meth:`_sql.ColumnOperators.istartswith`,
:meth:`_sql.ColumnOperators.iendswith`, which produce case-insensitive
LIKE compositions (using ILIKE on PostgreSQL, and the LOWER() function on
all other backends) to complement the existing LIKE composition operators
:meth:`_sql.ColumnOperators.contains`,
:meth:`_sql.ColumnOperators.startswith`, etc. Huge thanks to Matias
Martinez Rebori for their meticulous and complete efforts in implementing
these new methods.

Fixes: #3482
Closes: #8496
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/8496
Pull-request-sha: 7287e2c436959fac4fef022f359fcc73d1528211

Change-Id: I9fcdd603716218067547cc92a2b07bd02a2c366b