GCP Secrets Manager native support: Runner support

Proposal

This proposal is for adding a new CI keyword to configure secrets integration with GCP Secrets Manager. The CI keyword is implemented in #428401 (closed).

To GitLab Runner, the job payload will be as follows:

{
  "secrets": {
    "DATABASE_PASSWORD": {
      "gcp_secrets_manager": {
        "name": "my-project-secret",
        "version": "1",
        "server": {
          "project_number": "1234",
          "workload_identity_pool_id": "pool-id",
          "workload_identity_provider_id": "provider-id",
          "jwt": "jwt encoded GCP_SM_ID_TOKEN"
        }
      }
    }
  }
}

Based on this payload, the runner will need to perform the following tasks:

1. Authenticate with GCP Secret Token Service (sts) to obtain a bearer token scoped to the principal associated to the jwt claims.
2. Use the obtained bearer token to make an Access Secret Version request to GCP Secrets Manager. Doc
3. Create a job variable by the given name (e.g DATABASE_PASSWORD) with the secret payload obtained from GCP Secrets Manager.