Add lease to runs.

[mitchdenny]

This PR adds a PowerShell script that adds a retention lease to a pipeline run. Because you can only have one lease it first checks to see if a lease exists already, and if it does it removes it and adds it again for the specified validity period (there doesn't appear to be a patch operation according to the REST API docs).

[mitchdenny]

Closes: Azure/azure-sdk#1059

[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

[weshaggard]

Looks reasonable.

[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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