🐛 [EXTENSION_NAME_HERE] allow schedule email

[haitrungpham]

[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 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