[feature request] Send custom message when client send heartbeat

## Issue Description

Type: *feature request*

### Describe what happened (or what feature you want)

When sentinel-transport send heartbeat to dashboard's Endpoint, it can send user's custom messages.

### Describe what you expected to happen

User can custom information like follow:

```properties
group = basic
app = order
```

When sentinel-transport send heartbeat, it can add those messages.

### Tell us your environment

Sentinel Version: 1.8.1

### Anything else we need to know?

Start by tracing the sentinel-transport heartbeat sent to the dashboard's Endpoint and how the user's configuration is represented. Define the supported custom properties, such as group and app, and verify that the heartbeat includes them in the resulting message.

Written by the indexing model from the issue text.