ensure datatype roundtrips for JSON dialects

ensure datatype roundtrips for JSON dialects

Improved the behavior of JSON accessors :meth:`.JSON.Comparator.as_string`,
:meth:`.JSON.Comparator.as_boolean`, :meth:`.JSON.Comparator.as_float`,
:meth:`.JSON.Comparator.as_integer` to use CAST in a similar way that
the PostgreSQL, MySQL and SQL Server dialects do to help enforce the
expected Python type is returned.

The :meth:`.JSON.Comparator.as_boolean` method when used on a JSON value on
SQL Server will now force a cast to occur for values that are not simple
`true`/`false` JSON literals, forcing SQL Server to attempt to interpret
the given value as a 1/0 BIT, or raise an error if not possible. Previously
the expression would return NULL.

Fixes: #11074
Change-Id: I5024b78ec2fa6b61a9c6ee176112f1b761eeab98