Add support for timeout in process termination handling

[JanKrivanek]

Background:

related to dotnet/templating#4799

It would be helpfull to be able to specify optional maximum timeout for handling the process termination - in order to prevent unwanted 'unkillable' command - especially for cases where command builder and and actual implementation of the command have different maintainers (e.g. a case of dotnet sdk).

The process that don't repond to Ctrl-C can still be killed by other means - but it may create unpleasant and inconsistent experience to users. The idea is to have top level handler that gives command chance to perform cleanup in correct way, but bail out if they do not return in timely manner (e.g. due to not using the cancellation patter at all).

If the optinal timeout is not specified, the original behavior remains (no timeout enforced).

[JanKrivanek]

@jonsequitur thank you for the initial feedback (should be addressed now). Are there any further unaddressed concerns or suggestions?

[jonsequitur]

@jonsequitur thank you for the initial feedback (should be addressed now). Are there any further unaddressed concerns or suggestions?

I suggested a couple of test renames but feel free to adjust if you have something more precise. The goal is to embed the expectations about the scenario in the test name, like a spec.

[jonsequitur]

Is this test redundant with the pre-existing test above?

[JanKrivanek]

It is redundand with the current CancelOnProcessTermination implementation. But it guards the possibility of contract/behavior change on explict passing of null as timeout.
I can remove it if you feel it's superfluous.

[JanKrivanek]

@jonsequitur Are there any further unaddressed concerns or suggestions?