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.
gatt: Introduce helper for searching for services

This patch adds the btd_gatt_get_services() helper which searches for
services declarations with a specific UUID in an attribute database.
gatt: Introduce helper for searching for characteristics

This patch adds the btd_gatt_get_chars_decl() helper which searches for
characteristics from a service with a specific UUID in an attribute
database.
gatt: Introduce helper for searching descriptors

This patch adds the btd_gatt_get_char_desc() helper which searches for
a characteristic descriptor with a specific UUID in an attribute
database.
gatt: Introduce helper for searching for characteristics' values

This patch adds the btd_gatt_get_char_value() helper which searches
for the value of a specific characteristic in an attribute database.