feat: expose the `external` field in credentials resource

[trois-six]

Description

Add the field external to be able to configure external credentials.

Motivation and Context

Needed by a customer.

How Has This Been Tested?

In production.

Screenshots (if appropriate)

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist

• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes if appropriate.
• [x] All new and existing tests passed.

[welcome-to-palo-alto-networks[bot]]

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[welcome-to-palo-alto-networks[bot]]

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