feat: Ability to programmatically start `ion-refresher`

# Feature Request

**Ionic Info**
```
Ionic:
ionic (Ionic CLI) : 4.4.0
System:
NodeJS : v8.9.4
npm : 4.2.0
OS : Windows 10
```

**Describe the Feature Request**
As it was possible in Ionic 3.9.2, I would like to have a method to programaticaly begin refresh of a Refresher. I use it on every listPage to automatically load my data and show user that he can pulldown to refresh data

**Describe Preferred Solution**
A method on Refresher object which do the job.

**Describe Alternatives**
Here is my Ionic 3.9.2 code to do this

````
public IonRefreshOpen(refresher: Refresher) {
this._refresher = refresher;
this._refresher._top = `${refresher.pullMin}px`;
this._refresher._beginRefresh();
}
````

Thanks for your answers

Start by locating the current ion-refresher/Refresher implementation and its existing _beginRefresh and pullMin usage. Compare that path with the Ionic 3.9.2 behavior described here, then determine the public API and verification needed so a programmatic refresh visibly begins and loads data.

由索引模型根据 Issue 内容生成。