instance: fix network interface plan modifier (#634)

The instance `mac_address` is also a computed attribute, so it needs to
be set to `unknown` if the network interface is recreated.

I tried to create a reflect-based approach to handle this automatically,
but the code was a bit too much for what it is doing. I don't expect we
would change the schema too often, so I found a more explicit approach
better. We should reconsider this decision If we find ourselves running
into problems keeping this function updated though.

Author: lgfa29

internal/provider/resource_instance.go

@@ -2873,6 +2873,7 @@ func (v instanceNetworkInterfacesPlanModifier) PlanModifySet(
 		if nic.Hash() != stateNIC.Hash() {
 			plan[i].ID = types.StringUnknown()
 			plan[i].IPAddr = types.StringUnknown()
+			plan[i].MAC = types.StringUnknown()
 			plan[i].Primary = types.BoolUnknown()
 			plan[i].TimeCreated = types.StringUnknown()
 			plan[i].TimeModified = types.StringUnknown()