refactor(platform): Refactoring ES Utils, adding EXISTS condition support to Filter Criterion

[jjoyce0510]

Summary

In this PR, we support a new filter condition inside Criterion.pdl for checking whether a field EXISTS in elasticsearch. We also add some unit tests to ESUtils.java and refactor to place private methods below public ones.

Change summary

1. Refactor ESUtils.java for readability
2. Support EXISTS operation inside Condition.pdl
3. Add unit tests to confirm that ES Filters are consistently constructed as expected in various cases

Why make this change? This unlocks the ability to easily support EXISTS style filters from the UI and elsewhere against particular index fields.

Status

Ready for review

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)
• Docs related to the changes have been added/updated (if applicable). If a new feature has been added a Usage Guide has been added for the same.
• For any breaking change/potential downtime/deprecation/big changes an entry has been made in Updating DataHub