[test proxy] Move migration script to eng/common

[timovv]

For ease of use when migrating over to the new assets repo.

Cc @scbedd @HarshaNalluru

[azure-sdk]

The following pipelines have been queued for testing:
java - template
java - template - tests
js - template
net - template
net - template - tests
python - template
python - template - tests
You can sign off on the approval gate to test the release stage of each pipeline.
See eng/common workflow

[scbedd]

@timovv this looks great. I didn't see any places where you didn't update the location of the script.

Thank you for moving this, now approve the first step of the eng/common process, which will create the sync PRs.

On those same PRs, we will want to add little commits to update documentation referencing the old location of this script. I will help with that, but let me know when those PRs are created.

[azure-sdk]

The following pipelines have been queued for testing:
java - template
java - template - tests
js - template
net - template
net - template - tests
python - template
python - template - tests
You can sign off on the approval gate to test the release stage of each pipeline.
See eng/common workflow

[ghost]

Hello @azure-sdk!

Because this pull request has the auto-merge label, I will be glad to assist with helping to merge this pull request once all check-in policies pass.

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