When implicitly registering the credential secret is not stored

In the create for the MQTT connection when using the pairing token, the device is registered, but the credential secret is not stored in persistent memory.
This will create an error upon reconnection of the same device to Astarte. As the credential secret will not be found, a second registration will be attempted. Resulting in an error from Astarte.

https://github.com/astarte-platform/astarte-device-sdk-cpp/blob/6734124e25edc80c8706247f421ce93aa2b4fd8d/src/mqtt/connection.cpp#L269-L277

Start at src/mqtt/connection.cpp lines 269-277 and trace the create flow when an MQTT connection uses a pairing token. Reconnect the same device and verify that the credential secret is persisted and reused, without attempting a second registration with Astarte.

Written by the indexing model from the issue text.