Add server processes fact

[hjoukl]

Is your feature request related to a problem? Please describe

I needed to get a process list as a fact in ad-hoc execution mode and couldn't find a (pyinfra built-in) way to do that.

Describe the solution you'd like

Add the possibility to get at the processes of a target host. Maybe a fact server.Processes?

In fact (pun intended ;-)) there's a fact windos.Processes that looks similar(?), couldn't try it.

Btw pyinfra is really really great! Such a joy to use code rather than heaps of YAML that more often than not pile up to just a form of bad + complicated, degenerated code.

Best regards Holger