Add backend for Account Ownership Verification PIN feature

What does this MR do and why?

Related to #233164

This merge request implements the backend API endpoints,services, and User model methods for the Account Ownership Verification PIN feature. It adds the ability for users to generate and retrieve a temporary security PIN, and for administrators to retrieve a user's PIN for verification purposes.

The Account Ownership Verification PIN feature is needed to enhance security during support interactions. It provides a secure method for verifying account ownership without relying on sensitive personal information.

The pin exists in Redis only for a limited time (currently 24h).

1. Create Support PIN (POST /api/v4/user/support_pin):

sequenceDiagram
    participant User
    participant API
    participant CreateService
    participant Redis

    User->>API: POST /api/v4/user/security_pin
    API->>API: Authenticate User
    API->>CreateService: Execute
    CreateService->>CreateService: Generate PIN
    CreateService->>Redis: Store PIN with TTL
    Redis-->>CreateService: Confirm Storage
    CreateService-->>API: Return PIN and Expiry
    API-->>User: Return PIN and Expiry

2. Retrieve User's Own Support PIN (GET /api/v4/user/support_pin)

sequenceDiagram
    participant User
    participant API
    participant RetrieveService
    participant Redis

    User->>API: GET /api/v4/user/security_pin
    API->>API: Authenticate User
    API->>RetrieveService: Execute
    RetrieveService->>Redis: Get PIN
    Redis-->>RetrieveService: Return PIN (if exists)
    RetrieveService-->>API: Return PIN and Expiry
    API-->>User: Return PIN and Expiry (or Not Found)

3. Admin Retrieve User's Support PIN (GET /api/v4/users/:id/support_pin)

sequenceDiagram
    participant Admin
    participant API
    participant RetrieveService
    participant Redis

    Admin->>API: GET /api/v4/users/:id/support_pin
    API->>API: Authenticate Admin
    API->>API: Validate User ID
    API->>RetrieveService: Execute
    RetrieveService->>Redis: Get PIN for User ID
    Redis-->>RetrieveService: Return PIN (if exists)
    RetrieveService-->>API: Return PIN and Expiry
    API-->>Admin: Return PIN and Expiry (or Not Found)

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Create a Support PIN for the authenticate user: curl -X POST -H "PRIVATE-TOKEN: glpat-XXXXXXXXXXXXXXXXX" "https://gitlab.example.com/api/v4/user/support_pin"
2. Retrieve the current authenticated user's Support PIN: curl -X GET -H "PRIVATE-TOKEN: glpat-XXXXXXXXXXXXXXXXX" "https://gitlab.example.com/api/v4/user/support_pin"
3. As admin, retrieve a specific user's Support PIN: curl -X GET -H "PRIVATE-TOKEN: glpat-XXXXXXXXXXXXXXXXX" "https://gitlab.example.com/api/v4/users/123/support_pin"