Add admin page to show all duplicates

There are scenarios when it's possible to have duplicates for unique fields.
Scenario #1: The user has an admin role to skip validation.
Scenario #1: There were duplicates for the field before enabling configurations.

It should be possible to review all duplicates for admin users.
Solution:
1. Create a new permission to access the admin page.
2. Create a new /admin/config/content/unique-fields/duplicates route.
3. Implement a controller that should show a table with all duplicates.
It should have the following columns: field name, value, usages (links to related node/term/user).