Integrate Role and Permission Management from `solidus_user_roles` (solidus_core step)

[rainerdema]

Summary

Fixes #5105

This PR integrates role and permission management capabilities from the solidus_user_roles gem directly into Solidus, providing greater flexibility and control over user roles and permissions.

Major changes include:

1. Permission Sets: A new Spree::PermissionSet model has been added to the database schema. This model represents sets of permissions that can be assigned to roles. Each permission set has a name and a set identifier.

2. Role Permissions: A new Spree::RolePermission model has been added to the database schema. This model represents the association between roles and permission sets.

3. Data Migration: Pull Request This rake task creates Spree::PermissionSet records for each existing permission set subclass in Spree::PermissionSets::*, and Spree::RolePermission records for each role's associated permission sets. (It is designed to work with both new installations and existing Solidus stores that have custom roles and permissions).

bin/rails solidus:import_existing_permission_sets

4. ~Backend interface: closed~

5. ~Admin interface: Will only be merged into nebulab/admin branch~

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[elia]

I'm putting this on hold until we figure out a few things on how it will be used in the new admin dashboard.