Expose store interface to FFI
Expose stroage.Store interface to FFI by using an interface (mobilemodels.Store) that inherits from it.
The exposed FFI interface lives in the mobilemodels package which gets taken into account for the JNI code generation by gomobile. This way we don't need to expose the whole storage directory, which would result in unused generated Java code and which partly would even fail to run (such as generated Java bindings for the Desktop specific default store implementation)
This is part of the APIv5 integration into Android bitmask_android#9185
(the actual code generation happens here https://0xacab.org/leap/android_libs/bitmask-core-android/-/blob/main/build_core.sh?ref_type=heads#L122 ff., as you can see there the directory bitmask-core/mobile/mobilemodels is still missing on main, but I'll create a separate MR for that soon.)