Feature Request: Add Setting to Set a Custom User Agent

**Feature Request**

Option to set the User Agent the InAppBrowser uses.

It would be nice if you could set the User Agent you want sent with requests as one of the options passed when opening the InAppBrowser.

I know you can change it in config.xml for the entire Cordova app, but we just want the requests sent in the InAppBrowser to be a custom user agent string.

Start by locating how options passed when opening the InAppBrowser are handled in the Java, Objective-C, and JavaScript parts of the plugin, and compare that with the app-wide config.xml User Agent setting. Done means callers can provide a custom User Agent for InAppBrowser requests without changing the Cordova app's global setting; verify the behavior on the supported mobile platforms.

Written by the indexing model from the issue text.