2478 datachecks for unknown types

[MichaelFu512]

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Pull Request Description

Added a datacheck that checks if the amount of unknowns per dataset isn't more than 50%.

Closes #2478

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After creating the pull request: in order to pass the release_notes_updated check you will need to update the "Future Release" section of docs/source/release_notes.rst to include this pull request by adding :pr:123.

[codecov[bot]]

Codecov Report

Merging #3692 (672f549) into main (de675e9) will decrease coverage by 0.1%. The diff coverage is 76.1%.

@@           Coverage Diff           @@
##            main   #3692     +/-   ##
=======================================
- Coverage   99.7%   99.6%   -0.0%     
=======================================
  Files        339     341      +2     
  Lines      34239   34284     +45     
=======================================
+ Hits       34108   34142     +34     
- Misses       131     142     +11     

Impacted Files	Coverage Δ
.../data_checks_tests/test_unknown_type_data_check.py	63.0% <63.0%> (ø)
evalml/data_checks/unknown_type_data_check.py	93.8% <93.8%> (ø)
evalml/data_checks/__init__.py	100.0% <100.0%> (ø)
evalml/data_checks/data_check_message_code.py	100.0% <100.0%> (ø)

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