MINIFICPP-2703 Update GCP extension to use unified credentials

[lordgamez]

https://issues.apache.org/jira/projects/MINIFICPP/issues/MINIFICPP-2703

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[szaszm]

Is this a behavior change that users need to be aware of?

[lordgamez]

Is this a behavior change that users need to be aware of?

From what I gathered there shouldn't be changes in the behavior of the GCP processors. The only change I saw is that when default credentials are used we cannot check them if they are valid in schedule time, so if they are invalid it will only be realized in processor trigger time.