Update all DataCheck / DataChecks validate method to include a “action” key in returned dictionary

[angela97lin]

Closes #1880

Note that this does not actually populate the actions returned. That'll be done in #1881 to prevent a PR with too many logic changes + API changes. This just updates the API!

Internally, we were returning messages--updated this to results (open to a better name if anyone has one) to indicate that its no longer just messages that we're returning from validate :)

[codecov]

Codecov Report

Merging #1916 (3396057) into main (d0b6101) will not change coverage.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##             main    #1916   +/-   ##
=======================================
  Coverage   100.0%   100.0%           
=======================================
  Files         267      267           
  Lines       21699    21700    +1     
=======================================
+ Hits        21693    21694    +1     
  Misses          6        6           

Impacted Files	Coverage Δ
...ests/data_checks_tests/test_sparsity_data_check.py	100.0% <ø> (ø)
...ts/data_checks_tests/test_uniqueness_data_check.py	100.0% <ø> (ø)
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_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)
evalml/data_checks/multicollinearity_data_check.py	100.0% <100.0%> (ø)
... and 16 more

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[bchen1116]

Nice! Changes look good to me!