[CERTTF-457] feat: Github action for retrieving data from multi-device jobs

[boukeas]

Description

This PR introduces a Testflinger Github action that polls a multi-device job (receiving its ID as its input) and returns the job IDs and machine IPs of its child jobs as a JSON string.

Resolved issues

Resolves CERTTF-457.

Tests

The workflow below was used to submit a multi-device job without polling, use the poll-multi action to retrieve the data for the child jobs, and then display that data and confirm it's valid JSON.

name: Temporary workflow for testing multi-device polling action
on:
  push:
  workflow_dispatch:

jobs:
  submit-and-poll:
    name: Submit multi-device job and poll it for its data
    runs-on: [self-hosted, testflinger]
    steps:

    - name: Submit job to Testflinger
      id: submit
      uses: canonical/testflinger/.github/actions/submit@main
      with:
        poll: false
        job: |
          job_queue: multi-1
          name: multi-device-job
          output_timeout: 43200
          provision_data:
            jobs: 
            - job_queue: 202407-34216
              name: dell-xps-13-9350-0cc9-c34216
              reserve_data:
                ssh_keys:
                - lp:boukeas
                timeout: 600
            - job_queue: 202008-28168
              name: hp-eliteone800-g627-all-in-one-pc-c28168
              reserve_data:
                ssh_keys:
                - lp:boukeas
                timeout: 600
    - name: Retrieve multi-device job data
      id: poll
      uses: canonical/testflinger/.github/actions/poll-multi@CERTTF-457-multi-device-polling
      with:
        job-id: ${{ steps.submit.outputs.id }}
    
    - name: Verify
      shell: bash
      env:
        JOBS: ${{ steps.poll.outputs.jobs }}
      run: |
        echo $JOBS
        echo $JOBS | jq

Here is a successful test run. This is the output of the action:

{
  "7fcc82b1-1ab8-4fcc-9e3f-b0f9afbcec49": "10.102.182.56",
  "52fa5098-eb7d-4084-b9b0-b830214c089f": "10.102.161.241"
}

A syntactically incorrect version of the job was also submitted, to test that the action detects provisioning (i.e. allocation) failures for multi-device jobs.