Update data checks to return a dictionary

[angela97lin]

Continuation of #1444, closes #1325

Rather than returning the DataCheckMessage classes themselves, we now return the dictionary version so that validate returns:

{ "errors": [{"message": "message here", "data_check_name": "name here", "level": "error"}],
 "warnings": [#same here]
}

[codecov]

Codecov Report

Merging #1448 (a4a8d72) into main (f54abd3) will increase coverage by 0.1%.
The diff coverage is 100.0%.

@@            Coverage Diff            @@
##             main    #1448     +/-   ##
=========================================
+ Coverage   100.0%   100.0%   +0.1%     
=========================================
  Files         220      220             
  Lines       14672    14699     +27     
=========================================
+ Hits        14665    14692     +27     
  Misses          7        7             

Impacted Files	Coverage Δ
evalml/data_checks/default_data_checks.py	100.0% <ø> (ø)
evalml/automl/automl_search.py	99.7% <100.0%> (-<0.1%)	⬇️
evalml/data_checks/class_imbalance_data_check.py	100.0% <100.0%> (ø)
evalml/data_checks/data_check.py	100.0% <100.0%> (ø)
evalml/data_checks/data_check_message.py	100.0% <100.0%> (ø)
evalml/data_checks/data_checks.py	100.0% <100.0%> (ø)
evalml/data_checks/high_variance_cv_data_check.py	100.0% <100.0%> (ø)
evalml/data_checks/highly_null_data_check.py	100.0% <100.0%> (ø)
evalml/data_checks/id_columns_data_check.py	100.0% <100.0%> (ø)
evalml/data_checks/invalid_targets_data_check.py	100.0% <100.0%> (ø)
... and 14 more

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[dsherry]

LGTM! Left one suggestion but not blocking