fix: use semantic instead of literal comparison in DefaultEqualityTester

[mars-lan]

This fixes #1663
Also change the signature of EqualityTester to compare only DataTemplates

Checklist

• The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• Links to related issues (if applicable)
• Tests for the changes have been added/updated (if applicable)
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[keremsahin1]

Can we add a test case to https://github.com/linkedin/datahub/blob/master/metadata-dao-impl/ebean-dao/src/test/java/com/linkedin/metadata/dao/EbeanLocalDAOTest.java#L125 for this new fix?

[mars-lan]

Actually upon closer inspection, we should probably use rest.li's DataMapUtils instead of messing with the codecs directly: https://linkedin.github.io/rest.li/how_data_is_serialized_for_transport

Will discard this PR and try a different approach instead.

[mars-lan]

Actually DataMapUtils is in the internal namespace (com.linkedin.restli.internal.server.util) so probably not a good idea to depend on it. It's also doing exactly the same JSON serialization/deserialization in our RecordUtils without special coercion fix-up. Look like doing semantic equality testing is still the most appropriate fix in this case. Re-opening the PR with added unit test.