Configure Processes with Subsystems

[stuhood]

There are a few different usecases for per-Process configuration:

1. temporarily disabling caching (the equivalent of --cache[-scope]-ignore)
2. configuring timeouts in a uniform way that is transparent to the engine and allows for explaining how to adjust them.
3. allowing a user to specify additional arguments on a temporary basis (this one is slightly scary, but useful for debugging)

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This would involve at least two components:

• Support for registering scoped-Subsystems in rules.
• Add an optional Subsystem reference to Process that would be consumed for timeout and cache values.

Open questions:

• how this relates to Harden PATH setting in pex runs #9760 and Platform/Environment-specific subsystems #7735.
• whether this is one subsystem or two: preserving the literal --cache[-scope]-ignore syntax would be great, but the other options related to processes are not cache related... ie: --process[-scope]-timeouts.
• it's possible that a process Subsystem would not be scoped... rather, subclassed. Because an unscoped --process-extra-args would not make sense in general.

[stuhood]

The motivation for this issue has probably been removed by per-Environment configuration.