floss: Add GATT topshim

This adds topshim layer for floss Rust to FFI with bt_gatt.h. In this
patch only btgatt_client_interface_t and btgatt_server_interface_t are
shimmed, and the remaining BleScannerInterface and
BleAdvertiserInterface will be added separately using cxx.

Bug: 193916778
Tag: #floss
Test: Build floss on Linux

Change-Id: Id2857d7674222e1dccc906bf002d479c3af6c39f