mergify: auto-assign PR owner

[v1v]

What does this PR do?

Auto-assign the PR owner if the PR is not assigned yet

See https://docs.mergify.com/actions/assign/

Why is it important?

See https://github.com/elastic/beats/discussions/29991

A similar implementation was done sometime ago -> https://github.com/elastic/apm-pipeline-library/blob/ff50b3b5f4fab47b89d61183f1f3d1bb58b8795a/.mergify.yml#L49-L57 for another project

[mergify]

This pull request does not have a backport label. Could you fix it @v1v? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[elasticmachine]

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• Start Time: 2022-02-08T17:33:23.249+0000

• Duration: 22 min 7 sec

Test stats 🧪

Test	Results
Failed	0
Passed	3
Skipped	0
Total	3

💚 Flaky test report

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