Reducing the memory consumption of the `PushJobAgent`

[aldbr]

The objective is to enhance the exploitation of HPCs with no external connectivity in DIRAC. The current workflow is limited:

• The PushJobAgent only works if you use the dirac-jobexec executable.
• The PushJobAgent supports a very limited number of jobs in parallel (~150 jobs would consume ~50GB of memory on your DIRAC server).

I would like to greatly reduce these limitations by deploying a series of PRs:

• [X] split the JobWrapper.execute() method into 3 sub methods (preProcess, process, postProcess) to better isolate operations involving communications with the external (DIRAC) services from the payload itself. (https://github.com/DIRACGrid/DIRAC/pull/7460)
• [x] introduce the JobWrapperOfflineTemplate that solely executes the process method. (https://github.com/DIRACGrid/DIRAC/pull/7529)
• [ ] adapt the PushJobAgent to these changes so that it submits a JobWrapperOfflineTemplate directly to a remote CE. For a while, both the "traditional" and the new approaches will be supported. (https://github.com/DIRACGrid/DIRAC/pull/7587)

https://github.com/DIRACGrid/DIRAC/pull/7422 contains a overview of the full picture if you are interested in (will not be merged).