Custom NbToggleComponent icon

### Issue type

**I'm submitting a ...** (check one with "x")

* [ ] bug report
* [x] feature request

### Issue description

**Current behavior:**

Currently, the `NbToggleComponent` icon is hardcoded to `checkmark-bold-outline`

```html



```

**Expected behavior:**

Usage with custom icons, as seen in other Nebular components

```html

```
### Other information:

**npm, node, OS, Browser**
```
Node, npm: `v13.8.0` and `6.14.5`
OS: macOS (Catalina)
Browser: Chrome
```

**Angular, Nebular**
```
Angular: 9.1.7
Nebular: 5.10
```