create Java bindings with gomobile

• to integrate bitmask-core in Android, we need to verify gomobile exposes the API correctly:
• move swagger client out of internal since gomobile will fail to generate the JNI if internal packages are referenced from functions that are going to be exposed
• decide to export a single mobile package that contains the API vs. exporting all public methods in directory pkg
• we need to add bitmask-core to https://0xacab.org/leap/android_libs/bitmaskcore (which should be renamed)