mobile API improvements

This MR is related to bitmask_android#9185 (closed), intending to provide the functionality to setup a new provider. I've refactored the API so that we can pass golang models to the Java world, which removes the necessity to write additional model classes in Java. Whenever the swagger definition for the gomodel change, these changes will automatically be available in Java as well.

Unfortunately gomobile doesn't support yet passing slices via the FFI, which makes it required to write thin wrappers around the bridge and gateways arrays returned by the API. This is a temporary solution until https://github.com/golang/go/issues/13445 has been fixed - and it looks like there is some movement to get this issue resolved.

Moreover this MR adds the ability to fetch the provider.json and all available bridges (if that endpoint is enabled by the provider), which can be part of the provider setup which I want to solve with bitmask_android#9185 (closed)