gatt: Assign write callback for external services

gatt: Assign write callback for external services

This patch adds the callback for writing the external characteristic
value. Internally, GDBusProxy implementation calls Set method of the
"Value" property. At the moment, potential errors returned by the
external service implementation are being ignored.

The ATT operation (Write Command or Request) is not exposed to the
external service implementation. GATT/GDBusProxy handles possible
DBus errors returned, and notify the core using the callbacks informed.
Write Command doesn't have callback since it doesn't return ATT errors.