🐛 [EXTENSION_NAME_HERE] allow schedule email

### [READ] Step 1: Are you in the right place?

Issues filed here should be about a feature request for a **specific extension in this repository**. To file a feature request that affects multiple extensions or the Firebase Extensions platform, please reach out to
[Firebase support](https://firebase.google.com/support/troubleshooter/report/features/) directly.

### [REQUIRED] Step 2: Extension name

N/A

### What feature would you like to see?

allow to schedule sending email at a specific time

### How would you use it?

Schedule time for sending email would be very useful in many situations: send notification at specific time for example

The issue names no extension, file, test, or entry point. Start by identifying the intended extension and clarifying the scheduling requirements; completion criteria are not defined in the report.

Written by the indexing model from the issue text.