[Az.Network] Add support for certificate based authentication connections on Vpn Gateway

[brandonv191]

Description

This PR adds certificate-based authentication support for VPN Gateway connections in Azure PowerShell. This enables users to configure VPN gateways to use certificate authentication instead of pre-shared keys, providing enhanced security for site-to-site VPN connections.
Changes include:

• New cmdlet New-AzVirtualNetworkGatewayCertificateAuthentication to create certificate authentication configuration objects
• Added -AuthenticationType and -CertificateAuthentication parameters to New-AzVirtualNetworkGatewayConnection and Set-AzVirtualNetworkGatewayConnection
• Added -UserAssignedIdentityId parameter to Set-AzVirtualNetworkGateway for managed identity configuration
• Added scenario tests for certificate authentication
• Updated help documentation with practical examples

Mandatory Checklist

• Please choose the target release of Azure PowerShell. (⚠️Target release is a different concept from API readiness. Please click below links for details.)

  • General release
  • Public preview
  • Private preview
  • Engineering build
  • No need for a release

• Check this box to confirm: I have read the Submitting Changes section of CONTRIBUTING.md and reviewed the following information:

• SHOULD update ChangeLog.md file(s) appropriately
  • Update src/{{SERVICE}}/{{SERVICE}}/ChangeLog.md.
    • A snippet outlining the change(s) made in the PR should be written under the ## Upcoming Release header in the past tense.
  • Should not change ChangeLog.md if no new release is required, such as fixing test case only.
• SHOULD regenerate markdown help files if there is cmdlet API change. Instruction
• SHOULD have proper test coverage for changes in pull request.
• SHOULD NOT adjust version of module manually in pull request

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