Implement resource guard extension point and worker limiter

[cottsay]

The point of this extension point is to support extension points which affect what jobs can be run by the parallel executor at what times.

The primary motivation here is to support a GNU-style jobserver, but the extension point is designed such that it could use historical build data to determine which jobs to run to reduce the "long tail" of the build, improving resource utilization.

The limiting of how many total package-level jobs are allowed to run was moved into this extension, and can therefore now be disabled as any extension could.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 93.75000% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.78%. Comparing base (bb4e553) to head (31a3231).

Files with missing lines	Patch %	Lines
...olcon_parallel_executor/resource_guard/__init__.py	90.41%	6 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #43      +/-   ##
==========================================
- Coverage   97.38%   95.78%   -1.61%     
==========================================
  Files           4        7       +3     
  Lines         153      261     +108     
  Branches       41       52      +11     
==========================================
+ Hits          149      250     +101     
- Misses          1        7       +6     
- Partials        3        4       +1     

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