Backend Usages endpoint return `403 Forbidden` when token used is Provider Key instead of the Access Token.

GET /admin/api/services/2555417723913/backend_usages.json HTTP/1.1
Host: 3scale-supertest-admin.3scale.net:443
User-Agent: Go-http-client/1.1
Accept: application/xml
Authorization: Basic XXXX
Accept-Encoding: gzip

2022/03/04 16:01:30 HTTP/1.1 403 Forbidden

When an access token is used instead, the API works as expected. 

This issue was introduced in 3scale 2.11. In 3scale 2.10 backend usages endpoint could be used using Provider Key

This issue breaks 3scale operator capabilities to use Backends and Products with ProviderKeys. Which is the default, as the 3scale API does not allow to create and get Access Tokens using the ProviderKeys (always has been like that).

The workaround is that the customer manually creates access tokens in the UI and saves them in a secret to be read by the operator.

With the master token access, the tenant's provider key can be read, but not the access tokens associated to admin users.

Usually the current workflow is:

a) the customer installs 3scale and master access token is saved in `system-seed`

b) the customer creates a Tenant CR] with a reference to the system-seed secret

c) The operator saves the tenant's provider key in a secret

d) The customer creates Product CR and Backend CR with a reference to the tenant secret, which contains the provider key.

This workflow used to work for 3scale 2.10, but is broken in 2.11