Simplify graph_json output and fix bugs

[eccabay]

Closes #3038

Also fixes two bugs that prevented some pipelines from getting JSON serialized:

• Ensemble components store an entire estimator object as the "final_estimator" parameter. This code saves that estimator as "final_estimator_name" and "final_estimator_parameters" instead.
• JSON cannot serialize the np.int64 datatype, which is the type returned by Integer ranges for hyperparameter searches. This can be changed within the Integer() call, but that was too many places across the code base for this PR, so I left the fix within graph_json itself. In the future, however, it might be better practice to move over to having Integer generate int types instead of np.int64.

[codecov]

Codecov Report

Merging #3049 (a06865b) into main (a20dcd1) will decrease coverage by 0.8%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main   #3049     +/-   ##
=======================================
- Coverage   99.8%   99.0%   -0.7%     
=======================================
  Files        312     312             
  Lines      30232   30255     +23     
=======================================
- Hits       30144   29935    -209     
- Misses        88     320    +232     

Impacted Files	Coverage Δ
evalml/tests/automl_tests/test_automl.py	99.5% <ø> (-<0.1%)	⬇️
evalml/pipelines/pipeline_base.py	98.5% <100.0%> (+0.1%)	⬆️
evalml/tests/pipeline_tests/test_graphs.py	100.0% <100.0%> (ø)
evalml/automl/pipeline_search_plots.py	17.9% <0.0%> (-82.1%)	⬇️
...l/tests/automl_tests/test_pipeline_search_plots.py	25.6% <0.0%> (-74.4%)	⬇️
...ests/automl_tests/test_automl_search_regression.py	84.6% <0.0%> (-15.4%)	⬇️
.../automl_tests/test_automl_search_classification.py	89.8% <0.0%> (-10.2%)	⬇️
evalml/tests/automl_tests/test_automl_utils.py	90.6% <0.0%> (-9.4%)	⬇️
...lml/tests/automl_tests/test_iterative_algorithm.py	92.4% <0.0%> (-7.6%)	⬇️
evalml/automl/utils.py	98.4% <0.0%> (-1.6%)	⬇️
... and 2 more

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[ParthivNaresh]

Looks great thanks for addressing this!

[freddyaboulton]

@eccabay Thanks for adding this!

I still prefer letting users encode the output themselves as opposed to erroring out on some pipelines but I'm ok with the implementation you have now. If we decide to always try to encode to JSON, then it may be good to incorporate @angela97lin 's custom encoder so that we can catch tricky bools or other data types. That can happen in a follow up PR though.

[freddyaboulton]

This might technically be a breaking change?