feat: new experimental extension firestore-dialogflow-fullfilment

[pr-Mais]

Welcome to the Experimental Extensions repo! If this PR adds a new extension,
thank you, and please make sure you follow the steps below! Otherwise, you can
disregard this message.

• The new extension's id roughly follows the pattern
  [firebase-product]-[action]-[noun], like storage-resize-images
• The following match the extension's id:
  • The folder that contains the extension (my-new-extension/)
  • The name field of my-new-extension/functions/package.json
  • The name field of my-new-extension/extension.yaml
  • A new entry in the packages array of the lerna.json file in the
    repository's root
• I've added a generate-readme script to
  my-new-extension/functions/package.json that matches the other
  extensions in the repository
• I've run the following npm commands at the root of the repository:
  • npm install
  • npm run generate-readmes
  • npm run format