[MERGE] rating,project,livechat: refactor API for performance issue

This commit makes several improvement in order to speed up and clean
the rating API. No functionnal changes should be obsered.
Multiple point are covered here:

1/ introduces a parent.mixin: since we want to extract the satisfaction
percentage on parent rating model (project, team, livechat channel),
providing a mixin to standardize this computation in a performant and
lazy way seems a good optimization (more details in the concerned
commit).

2/ cleaning API: some methods needed to be private and/or renamed.

3/ Better integration in the business application: use the new parent
mixin, remove legacy code and fields, ...

Task-1903565

closes odoo/odoo#28459

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