Group push rules: extract shared logic from `PushRules` into a module

Implementation plan

1. Extract shared logic from PushRules into a module.
2. Include the new module in the new GroupPushRule model (https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/models/group_push_rule.rb).

Expectations

Create a shared module between PushRules and GroupPushRule using Group push rules: Create table `group_push_rules` (#498933 - closed) as a reference.