Add mergeOptions support for Firestore set operation

[TheNoumanDev]

Summary

• Adds mergeOptions property to Firestore set operation
• Supports merge: true and mergeFields: [...] options
• mergeFields takes priority if both are provided
• By default merge is false, when mergeOptions isn't defined for backward compatibility

The benefit of using merge is that if the document doesn't exist, it creates the new document with the given fields, but if the document already exists, it just updates the given field, while the rest remain the same.

Example Usage

# merge: true - merges all passed fields, preserves existing
myAPI:
  type: firestore
  path: users/user123
  operation: set
  mergeOptions:
    merge: true
  data:
    name: "John"
    email: "john@example.com"

# mergeFields - only writes specified fields useful with form fields data or api response.
myAPI:
  type: firestore
  path: users/user123
  operation: set
  mergeOptions:
    mergeFields:
      - name
      - updatedAt
  data:
    name: "John"
    email: "ignored"
    updatedAt: ${FieldValue.serverTimestamp()}

Once approved, then before merging i will add the docs, schema, and Kitchen Sink Example.

Schema PR: https://github.com/EnsembleUI/ensemble-web-studio/pull/1725
Docs: EnsembleUI/ensemble_docs#98 (comment)
Also updated Kitchen Sink Example