Link token to user ids

Hi @appleboy ,

here is a thought I would like to discuss...

In my experience, a recurring concern about push notifications is sending notifications to a particular user of your service.
To achieve that, you have to maintain the matching between **user id** and its **push notification tokens**.

So, to achieve that, I would need :
- a microservice to maintain the **user id**/**push token** matching
- an instance of GoRush to actually send the notifications

It would be really convenient to add that responsability to GoRush.

## New API proposal
### POST /api/session

> creates a mapping between user id, and its tokens

```json
{
"user_id": "xxxxxx",
"token": {
"platform": 1,
"token": "sdfsfdsf",
}
}
```

### POST /api/push/session

> sends notification to users

```json
{
"notifications": [
{
"user_ids": ["user_id_1", "user_id_2"],
"message": "Hello World Users !",
"title": "Gorush with HMS"
},
]
}
```

### Cleaning the mapping
In case of invalid **tokens** (expired, invalid...) we want to remove it from the session mapping.

Fortunately, when sending notifications with services like Firebase Cloud Messaging, it returns the list of invalid tokens used.

What do you think ?

No files or tests are identified. Start by reviewing GoRush's existing API and notification delivery paths, then clarify the user/token mapping, proposed endpoints, and invalid-token cleanup behavior. Done would require an agreed design and implementation scope.

Written by the indexing model from the issue text.