Added policy management cmdlets for Azure Attestation Service

[gkostal]

Description

This PR contains the code changes for approved Issue #450 which defines three new cmdlets for the Azure Attestion Service (currently in preview). These cmdlets enable management of the attestation issuance policy. The changes included in this PR include:

• Three new cmdlets
  • Get-AzAttestationPolicy
  • Set-AzAttestationPolicy
  • Reset-AzAttestationPolicy
• Scenario tests
• Scenario recordings
• Help files

All tests (automated and ad-hoc) are passing.

Checklist

• I have read the Submitting Changes section of CONTRIBUTING.md
• The title of the PR is clear and informative
• The appropriate ChangeLog.md file(s) has been updated:
  • For any service, the ChangeLog.md file can be found at src/{{SERVICE}}/{{SERVICE}}/ChangeLog.md
  • A snippet outlining the change(s) made in the PR should be written under the ## Upcoming Release header -- no new version header should be added
• The PR does not introduce breaking changes
• If applicable, the changes made in the PR have proper test coverage
• For public API changes to cmdlets:
  • a cmdlet design review was approved for the changes in this repository (Microsoft internal only)
  • the markdown help files have been regenerated using the commands listed here