Introduce new SKUs and Generations for VirtualNetwork VPN gateways.

[ritwikbasu]

Description

Introduces new high-throughput SKUs and generations for Virtual Network VPN gateways. The commandlet review for this change is https://github.com/Azure/azure-powershell-cmdlet-review-pr/issues/354.

Checklist

• [Y] I have read the Submitting Changes section of CONTRIBUTING.md
• [Y] The title of the PR is clear and informative
• [Y] 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
• [Y] The PR does not introduce breaking changes
• [Y] If applicable, the changes made in the PR have proper test coverage
• [Y] For public API changes to cmdlets:
  • [Y] 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

[ritwikbasu]

@erich-wang I have resolved your comments. Please check and let me know.