Add terraform import functionality to the "sdwan_attach_feature_device_template" resource

[cmohorea]

Suggestion:
Add importing functionality to the sdwan_attach_feature_device_template by retrieving variable values and adding then to the state.
It is a part of the process anyway when adding/changing variable later on.

Otherwise, in a current model, in brownfield, even if you have a config and you manually defined all the values, 1st "terraform apply" is a change, even if TF config is same as real (and potentially dangerous as we don't know if we matched values correctly or missed something)