[CI] Github Robot - Recheck support

[Aaron Conole]

Greetings DPDK Developers,

The UNH IOL, and the DPDK CI community at large, have developed a
mechanism to request a test recheck for failures.  This allows
patch authors or maintainers who suspect that a failing test is
really a byproduct of something unrelated (flaky test, network issue,
etc) to tell the various CI labs to restart their testing.

Each lab is responsible for turning on this feature.  The Github Actions
robot run by Red Hat now has learned to parse this format.  The test
name that the robot will key on is 'github-robot'.  So an example of
asking it for a recheck:

  Recheck-request: github-robot

Or if there are multiple labs:

  Recheck-request: iol-compile-amd64-testing, github-robot

Etc.

Some important notes:

 1. The Github Actions robot does not currently parse cover-letter
    comments.

 2. The Github Actions robot is currently only using the direct polling
    API for retrieving details - this limits it to only scanning more
    recent series.

 3. As with the UNH IOL implementation, we are looking into adding an
    option that will allow to re-apply to the current tip of tree.

 4. As with the UNH IOL look into adding a retest vetting process - if
    the current permissions become an issue.

Thanks,
Aaron