fix(graphql): Support querying Posts and Queries

[asikowitz]

I dislike this massive Constants.java file that just has hundreds of somewhat-organized constants. Feels like an enum in its own file would be significantly better. Then we could, for example, make a test to ensure ENTITY_TYPE_TO_NAME supports all defined entity types.

In any case, this should be an improvement, and let me query posts and queries

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.
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[chriscollins3456]

lol THANK YOU the lack of consistency in this file has always bothered me and i feel silly for not having made this change myself