Add Microsoft.PolicyInsights/attestations API (#12661)

* Attestations API

* Update definitionReferenceId description

* Update owner description

* Update readmes with new tag

* Move to 2021-01-01

* Update api-version

* Update owners

Author: pilor

CODEOWNERS

@@ -49,7 +49,7 @@
 /specification/operationalinsights/ @sw47
 /specification/operationsmanagement/ @dashimi16
 /specification/peering/ @rileymckenna
-/specification/policyinsights/ @bulentelmaci
+/specification/policyinsights/ @cheggert
 /specification/postgresql/ @qingqingyuan
 /specification/powerbidedicated/ @tarostok
 /specification/provisioningservices/ @kvish