Adding support for ConnectionMode parameter for Virtual Network Gateway Connections

[abhi7860]

Description

Added an optional parameter ConnectionMode to VNG Connections which can be set while creating or updating the connection.

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:
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  • 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)
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  • the markdown help files have been regenerated using the commands listed here