Webhook Sync on gmail 📨

[FelixMalfait]

Discussed in #3937

^{Originally posted by FelixMalfait February 13, 2024}
As of today the email sync is done through a cron that runs every 10 minutes.
We'll need to provide immediate sync through webhooks.

We'll start with gmail.

• Create an environment variables to store the topic. Document it in enviroment-variables.mdx. Default value = gmail
• Call the /watch endpoint to subscribe to account upon mailbox connection
• Create an endpoint to receive subscriptions events and import email (ref)
• Create the topic and push subscription in our Google Cloud Console
• Setup a cron to renew the /watch API call, configure it to run daily