Create label-external-issues.yml

[jarmak-nv]

Description

This PR adds the external issue labeler GHA. It is the same as in nv-morpheus/Morpheus.

Checklist

• [x] I am familiar with the Contributing Guidelines.
• [x] New or existing tests cover these changes.
• [x] The documentation is up to date with these changes.

[mdemoret-nv]

/merge