Provide new IPC requests in safe_app

1. Provide a new IPC request encoding function in `safe_app::ffi::ipc` - `encode_share_mdata_req`. It should work in the exact same way as others (e.g. encode_containers_req). Use the following function signature:

2. Add a new callback to the `decode_ipc_msg` function in `safe_app::ffi::ipc` with the following signature:

Call this callback when the incoming response from Authenticator decodes to `IpcMsg::Resp { resp: IpcResp::ShareMData(...), resp_id }`

3. Add corresponding tests in the `safe_app::ffi::ipc` module (follow existing tests for an example).