Add endpoint for retrieving Service Ping Ping payload

What does this MR do and why?

This MR adds new API endpoint which exposes the latest ServicePing payload.

If no payload was generated on the instance, it returns an empty body

In order to access payload user needs a PAT with service_ping_access scope, which was added in !140936 (merged)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Endpoint OpenAPI specs:

  "/api/v4/usage_data/service_ping":
    get:
      summary: Get the latest ServicePing payload
      description: This feature was introduces in Gitlab 16.8
      produces:
      - application/json
      responses:
        '200':
          description: Get the latest ServicePing payload
        '401':
          description: 401 Unauthorized
        '403':
          description: Forbidden
        '404':
          description: Not found
      tags:
      - usage_data
      operationId: getApiV4UsageDataServicePing

How to set up and validate locally

1. Generate SerivicePing payload Gitlab::Usage::ServicePingReport.for(output: :all_metrics_values) in rails console. Alternatively you can try to retrieve an empty payload to verify it works.
2. Create PAT with service_ping_access scope (available only for admins)
3. Verify Service ping can be downloaded via API call

curl -X GET --location "http://localhost:3000/api/v4/usage_data/service_ping?private_token=abc" \
    -H "Accept: application/json"

Related to #435351 (closed)