Adding new optional parameter to specify dns servers for Virtual Wan-P2SVpnGateway and P2SClients

[Nilambari]

Description

Currently, with VirtualWan-P2SVpnGateway creation, there is no way for customers to specify custom dns servers which they are planning to use. Once, customer can specify custom dns servers, it will get set on gateway(HubVNet) and will be passed to customer Point to site clients as well. So, Point to site clients can also use their dns server/s instead of azure dns. This feature is asked by many customers till now and we have helped them with updating it at backend till this feature releases in PowerShell and customer can add custom dns servers themselves.

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

[Nilambari]

Related PowerShell command let command let review :- https://github.com/Azure/azure-powershell-cmdlet-review-pr/issues/585 is approved.
Related merged swagger PR:- Azure/azure-rest-api-specs#9268

ToDo:- Need to add updated help files and recorded test session file.

[anton-evseev]

Please re-open when May release branch would be available