Custom VPN Client - add donation button

The provider should be able to define a build property with the url leading the user to the provider's donation page.

There should be an additional build property that defines if the donation option should be enabled or not.

If the latter property is set to true a donation entry will be shown in the navigation drawer.

If the provider enables the donation property without defining an url, the default LEAP/Bitmask donation url will be used.