🚀 Enhancement: Support for multiple notification token per account

### 🔖 Enhancement description

Currently, Appwrite Messaging requires notification targets to be unique and tied to a single user. This does not support apps where a user can log into multiple accounts on their device. With an app that supports multiple accounts, the same device is expected to receive notifications for all logged in accounts even though they might not be the active account. An example of this is Gmail, a user can log into multiple email accounts and receive notifications for all logged in accounts. This use-case is not supported by Appwrite Messaging APIs.


### 🎤 Pitch

Kindly consider allowing multiple push notification targets to be tied to more than a single user. It could probably be a setting that can be enabled/disabled.

### 👀 Have you spent some time to check if this issue has been raised before?

- [x] I checked and didn't find similar issue

### 🏢 Have you read the Code of Conduct?

- [x] I have read the [Code of Conduct](https://github.com/appwrite/.github/blob/main/CODE_OF_CONDUCT.md)