Firestore Document Add / Edit as JSON

Note: Before filing feature requests in this repo, please ensure that this is a feature request for the Emulator Suite UI. Emulator Suite and/or Firebase CLI feature requests should be filed under the firebase-tools repo here:
https://github.com/firebase/firebase-tools/issues/new/choose

**Is your feature request related to a problem? Please describe.**
When I manually add a new or edit a Document with several fields, I need to do it field by field.

**Describe the solution you'd like**
Possibility to add/edit a document as JSON

**Describe alternatives you've considered**
Add field by field

**Additional context**
Simplify Firestore data manipulation