feat: send Referer header on ios

## Feature Request

### Description

Hello, I am using a service that secures web apps using the HTTP Referer header. This service does allow the capacitor:// scheme which is default on iOS. I am having an issue were capacitor on iOS is not sending the header, even though it does send the Origin header and other headers. The service offers a js npm module which uses XMLHttpRequest. I am not having issues on Android.

### Platform(s)

iOS

### Preferred Solution

Add the HTTP Referer header to XMLHttpRequests.

### Alternatives

Overriding XMLHttpRequest to inject the header. (Haven't tried it, yet.)

### Additional Context

Attached safari debugger and found this:

```
GET /path HTTP/1.1
Accept: /
Origin: capacitor://localhost
Host: host.name.com
User-Agent: Mozilla/5.0 (iPhone; CPU iPhone OS 13_4 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Mobile/15E148
Accept-Language: en-us
Accept-Encoding: gzip, deflate, br
Connection: keep-alive
```