feat(graphql) Support exists operator in GraphQL Search API

[jjoyce0510]

Summary

Tiny PR for adding the exists operator to GraphQL API. This was confirmed to work locally, because we already support this in Condition.pdl on the Elasticsearch backend.

Tested queries:

query searchAcrossEntities {
  searchAcrossEntities(input:{
    types: [MONITOR],
    start: 0,
    count: 20,
    query: "*",
    orFilters: [
    {
      and: [
        { field: "executorId", condition: EXISTS }
      ]
    }
    ]
  }) {
    total
    start
   	searchResults {
      entity {
        ...on Monitor {
          urn
        }
      }
    }
  }

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

[chriscollins3456]

🎸