Add cpu and mem options for shared grader spawner

[jgwerner]

• Adds option to include requests and limits for both CPU and Memory
• Updates README with new env var settings

Closes #607 #509

[codecov[bot]]

Codecov Report

Merging #612 (e9f4b10) into main (16dfcd4) will not change coverage. The diff coverage is 66.66%.

@@           Coverage Diff           @@
##             main     #612   +/-   ##
=======================================
  Coverage   66.50%   66.50%           
=======================================
  Files          12       12           
  Lines         806      806           
=======================================
  Hits          536      536           
  Misses        270      270           

Impacted Files	Coverage Δ
src/illumidesk/illumidesk/spawners/hooks.py	0.00% <0.00%> (ø)
src/illumidesk/illumidesk/apis/nbgrader_service.py	60.56% <100.00%> (ø)
...illumidesk/illumidesk/apis/setup_course_service.py	62.06% <100.00%> (ø)

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