Rework verify agent OS step.

[mitchdenny]

This PR reworks the verify agent OS logic so that the agent pool and image are passed into a PowerShell script, and then is checked against a list of valid configuration entries.

[mitchdenny]

Blocked on this PR: Azure/azure-sdk-for-net#18518

[azure-sdk]

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[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]

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