Add support for characteristic descriptions

[Oskee-Wow-Wow]

Would be useful to have characteristic descriptions available, but the two current description calls return GATTLIB_NOT_SUPPORTED:

int gattlib_discover_desc_range(gatt_connection_t* connection, int start, int end, gattlib_descriptor_t** descriptors, int* descriptor_count) {
        return GATTLIB_NOT_SUPPORTED;
}

int gattlib_discover_desc(gatt_connection_t* connection, gattlib_descriptor_t** descriptors, int* descriptor_count) {
        return GATTLIB_NOT_SUPPORTED;
}

Thanks