Use default application credentails for Dialogflow instead of insecure private key

[Mistic92]

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• Bug report
• Feature request
• Documentation issue or request
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Expected Behavior

When using Dialogflow NLU or just integration, there should be used so called Default Application Credentials instead of insecure service account key.
This can be set localy just using gcloud and on GCP it is provided by runtime environment. Also google-auth-library handle environment variable GOOGLE_APPLICATION_CREDENTIALS by itself to no jwt should be used in that explicit way.

Issue is in v4 and v3 in this lines https://github.com/jovotech/jovo-framework/blob/v4/dev/integrations/nlu-dialogflow/src/DialogflowNlu.ts#L89

Current Behavior

Requirement of generating json key which is not secure practice.

Error Log

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Your Environment

• Jovo Framework version used: 4.x.x 3.x.x
• Operating System: