Add support for multi request polling

Summary

• Adds support for polling multiple endpoints
• If using multiple endpoints, does not poll completed endpoints (e.g. if 2 out of 5 endpoints return 200, it stops polling them).

Context

The Security Reports MR Widget request to poll multiple endpoints. Our current API does not have support for that. This MR adds a new fetch method which returns an array of functions, which will be used to create a separate poll for each of them.

Example usage

// extensions/security_reports/index.js

{
  enablePolling: true,

  fetchMultiData() {
    return [ 
      this.fetchReport(this.sastEndpoint, "SAST"),
      this.fetchReport(this.dastEndpoint, "DAST")
    ]
  },
 
  fetchReport(endpoint, reportType) {
      if (!endpoint) {
        return Promise.resolve();
      }

      return axios.get(endpoint).then((r) => ({ ...r, data: { ...r.data, reportType } }));
    },
}

Related to #361757 (closed)