gatt: Introduce helper for adding GATT services

This patch adds the btd_gatt_add_service() helper which adds a
GATT Service declaration to local attribute database.
gatt: Introduce helper for adding characteristics

This patch adds the btd_gatt_add_char() helper which adds a
characteristic (both declaration and value attributes) to local
attribute database.
gatt: Introduce helper for adding characteristic descriptors

This patch adds the btd_gatt_add_char_desc() helper which adds a
characteristic descriptor to local attribute database.
gatt: Introduce helper for removing services

This patch adds the btd_gatt_remove() helper which removes a
service (along with all its characteristics) from the local
attribute database.