Added AutoMLSearch object as another parameter in search callbacks

[christopherbunn]

For both start_iteration_callback and add_results_callback, the AutoMLSearch object passed in as the third parameter.

Resolves #1095

[codecov]

Codecov Report

Merging #1156 into main will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1156   +/-   ##
=======================================
  Coverage   99.91%   99.91%           
=======================================
  Files         195      195           
  Lines       11554    11554           
=======================================
  Hits        11544    11544           
  Misses         10       10           

Impacted Files	Coverage Δ
evalml/automl/automl_search.py	99.58% <100.00%> (ø)
evalml/tests/automl_tests/test_automl.py	100.00% <100.00%> (ø)
.../automl_tests/test_automl_search_classification.py	100.00% <100.00%> (ø)
...ests/automl_tests/test_automl_search_regression.py	100.00% <100.00%> (ø)

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[freddyaboulton]

@christopherbunn Looks good to me!