RBAC: implement role management endpoints

[leovalais]

Description and goal

Part of #8171

Implement the following routes:

No role control required

• GET /authz/list_roles ➡️ all application roles declared in the current configuration
• GET /authz/roles ➡️ the roles of the issuer of the request (available to every authenticated user)

On role = admin

• POST /authz/subject/:id/roles ➡️ grants some roles to a subject
• DELETE /authz/subject/:id/roles ➡️ removes some roles of a subject

Acceptance criteria

The endpoints exist.