Add externalIP attribute for network_interface

[yussufsh]

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Relates OR Closes #0000

Output from acceptance testing:

=== RUN   TestAccIBMPINetworkInterfaceBasic
--- PASS: TestAccIBMPINetworkInterfaceBasic (54.43s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/power   56.390s

=== RUN   TestAccIBMPINetworkInterfaceDataSourceBasic
--- PASS: TestAccIBMPINetworkInterfaceDataSourceBasic (43.78s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/power   45.948s

=== RUN   TestAccIBMPINetworkInterfacesDataSourceBasic
--- PASS: TestAccIBMPINetworkInterfacesDataSourceBasic (39.89s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/power   41.805s

[yussufsh]

These changes were tested on top of https://github.com/IBM-Cloud/terraform-provider-ibm/pull/6704/changes