Add List owners

[noyainrain]

Make it possible to grant multiple users ownership of a list.

[noyainrain]

Draft:

_O = TypeVar('_O', bound=Object)

class Owners(Collection[User]):
    """:ref:`Collection` of :ref:`Users` s who own the object.

    The object has at least one owner.

    Events:

    .. describe:: object-owners-grant

       Published when ownership of the object has been granted to a :ref:`User`.

    .. describe:: object-owners-revoke

       Published when ownership of the object has been revoked from a :ref:`User`.
    """

    def grant(self, user: User) -> None:
        """
        .. http:post:: /api/(resource-url)

           ``{user_id}``

           Grant ownership of the object to the :ref:`User` with *user_id*.

           Permissions: Object owners.
        """

    def revoke(self, user: User) -> None:
        """
        .. http:delete:: /api/(resource-url)/(id)

           Revoke ownership of the object from the :ref:`User` with *id*.

           Permissions: Object owners.
        """

class List:
    """
    .. describe:: owners

       List :ref:`ListOwners`.
    """

    owners: Owners

class CollectionEvent(Generic[_O], Event):
    """:ref:`Event` related to a :ref:`Collection`.

    .. describe:: item_id

       ID of the involved item.

    .. describe:: item

       Involved item.
    """

    item_id: str
    item: _O