Add sync_backref flag in a relationship

Add sync_backref flag in a relationship

Introduce :paramref:`_orm.relationship.sync_backref` flag in a relationship
to control if the synchronization events that mutate the in-Python
attributes are added.
This flag is implied in a ``viewonly=True`` relationship.
This supersedes the previous change #5149, that warned that ``viewonly=True``
relationship target of a back_populates or backref configuration would be
disallowed.

Fixes: #5237
Change-Id: I22c5ba28dcea22fc78a83e68e667140edffc515c