Update authentication validation steps

[jgwerner]

Segregate launch validation by steps as described in IMS Global's LTI 1.3 specification.

[codecov]

Codecov Report

Merging #168 into main will increase coverage by 0.95%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #168      +/-   ##
==========================================
+ Coverage   74.10%   75.05%   +0.95%     
==========================================
  Files          13       13              
  Lines         946      942       -4     
==========================================
+ Hits          701      707       +6     
+ Misses        245      235      -10     

Impacted Files	Coverage Δ
src/illumidesk/authenticators/constants.py	100.00% <100.00%> (ø)
src/illumidesk/authenticators/handlers.py	90.00% <100.00%> (-1.18%)	⬇️
src/illumidesk/authenticators/validator.py	90.09% <100.00%> (+9.54%)	⬆️

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[netoisc]

LGTM