KeyFuncDict regression fixes and dataclass fixes

KeyFuncDict regression fixes and dataclass fixes

adapt None-key warning for non-mapped attributes

Fixed multiple regressions due to :ticket:`8372`, involving
:func:`_orm.attribute_mapped_collection` (now called
:func:`_orm.attribute_keyed_dict`).

First, the collection was no longer usable with "key" attributes that were
not themselves ordinary mapped attributes; attributes linked to descriptors
and/or association proxy attributes have been fixed.

Second, if an event or other operation needed access to the "key" in order
to populate the dictionary from an mapped attribute that was not
loaded, this also would raise an error inappropriately, rather than
trying to load the attribute as was the behavior in 1.4.  This is also
fixed.

For both cases, the behavior of :ticket:`8372` has been expanded.
:ticket:`8372` introduced an error that raises when the derived key that
would be used as a mapped dictionary key is effectively unassigned. In this
change, a warning only is emitted if the effective value of the ".key"
attribute is ``None``, where it cannot be unambiguously determined if this
``None`` was intentional or not. ``None`` will be not supported as mapped
collection dictionary keys going forward (as it typically refers to NULL
which means "unknown"). Setting
:paramref:`_orm.attribute_keyed_dict.ignore_unpopulated_attribute` will now
cause such ``None`` keys to be ignored as well.

Add value constructors to dictionary collections

Added constructor arguments to the built-in mapping collection types
including :class:`.KeyFuncDict`, :func:`_orm.attribute_keyed_dict`,
:func:`_orm.column_keyed_dict` so that these dictionary types may be
constructed in place given the data up front; this provides further
compatibility with tools such as Python dataclasses ``.asdict()`` which
relies upon invoking these classes directly as ordinary dictionary classes.

Fixes: #9418
Fixes: #9424
Change-Id: Ib16c4e690b7ac3fcc34df2f139cad61c6c4b2b19