more extensibility for asc/desc

more extensibility for asc/desc

Improved the behavior of standalone "operators" like :func:`_sql.desc`,
:func:`_sql.asc`, :func:`_sql.all_`, etc. so that they consult the given
expression object for an overriding method for that operator, even if the
object is not itself a ``ClauseElement``, such as if it's an ORM attribute.
This allows custom comparators for things like :func:`_orm.composite` to
provide custom implementations of methods like ``desc()``, ``asc()``, etc.

Added default implementations of :meth:`.ColumnOperators.desc`,
:meth:`.ColumnOperators.asc`, :meth:`.ColumnOperators.nulls_first`,
:meth:`.ColumnOperators.nulls_last` to :func:`_orm.composite` attributes,
by default applying the modifier to all contained columns.  Can be
overridden using a custom comparator.

Fixes: #12769
Change-Id: I055ce79bf7ac31fb61d48bc3ab34799d42fb6336