Disable clang-tidy checking on files with prefix ext_

[YanzhaoW]

Requested from PR #896.

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Checklist:

• [x] Rebased against dev branch
• [x] My name is in the resp. CONTRIBUTORS/AUTHORS file
• [x] Followed the pull request guidlines and the Git workflow
• [x] Followed the seven rules of great commit messages

[YanzhaoW]

Ok, this test result successfully suppress the checks on files with ext_ prefix compare to this test.

[YanzhaoW]

@jose-luis-rs Ready to be merged.

[jose-luis-rs]

Thanks