[IMP] digest: add private methods to manage digest members

Purpose is to ease digest management and testing. Public methods still work
as before this commit, simply calling the private implementation. New private
methods allows to better manipulate subscribed users. Those will be used
notably as shortcuts in tests.

Task-2641394 (Digest emails sending improvement)
Task-2582128 (Digest onbarding and usage improvement)

X-original-commit: 3045c49f
Part-of: odoo/odoo#79877