add support for GPU request

[belforte]

start with a FAQ on how to do it via extraJDL
adding a configuration parameter in the crabConfig may be a lot of work since it
requires schema change and changing a zilion places in the REST APIs where the list
of all columns has to be given in correct order.

example on how to do it condorside:
https://ci-connect.atlassian.net/wiki/spaces/CMS/pages/80117822/Requesting%2BGPUs

[belforte]

FAQ added:
https://twiki.cern.ch/twiki/bin/view/CMSPublic/CRAB3FAQ#How_can_I_tell_CRAB_to_run_jobs

[belforte]

set priority to low since this should be sufficient for initial use. Will see depending on user adoption if worth to add one ad-hoc parameter. Or find some hack by which we can become more flexible like one `config.Debug.extraParams=[list of strings]' and solve forever the problem of adding configuration parameters.

[belforte]

now overridden by #6989