Add ability to freeze hyperparameters in AutoMLSearch

[bchen1116]

fix #1625

Design discussion here
Updated documentation here

[codecov]

Codecov Report

Merging #1676 (39c8551) into main (c321cae) will increase coverage by 0.1%.
The diff coverage is 100.0%.

@@            Coverage Diff            @@
##             main    #1676     +/-   ##
=========================================
+ Coverage   100.0%   100.0%   +0.1%     
=========================================
  Files         242      242             
  Lines       18942    19055    +113     
=========================================
+ Hits        18934    19047    +113     
  Misses          8        8             

Impacted Files	Coverage Δ
...lml/automl/automl_algorithm/iterative_algorithm.py	100.0% <100.0%> (ø)
evalml/automl/automl_search.py	99.7% <100.0%> (+0.1%)	⬆️
evalml/tests/automl_tests/test_automl.py	100.0% <100.0%> (ø)
...lml/tests/automl_tests/test_iterative_algorithm.py	100.0% <100.0%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c321cae...39c8551. Read the comment docs.

[chukarsten]

This looks pretty solid. I would certainly let someone else take a look at this, because I'm not extremely familiar with the source issue or what's driving this change. But the way you did it looks good.

[freddyaboulton]

@bchen1116 Looks great! I like how this handles all the different ways someone might want to "freeze" some hyperparams.

[ParthivNaresh]

Looks great, excellent test coverage