Gateway Load Balancer inconsistency fix

[matyang22]

review PR https://github.com/Azure/azure-powershell-cmdlet-review-pr/issues/1000

Description

`The new parameter was introduced here https://github.com/Azure/azure-powershell-cmdlet-review-pr/issues/898 but it also introduced some inconsistency.
This change is to fix inconsistency in 2 powershell commands.

One command(New-AzLoadBalancerBackendAddressPoolConfig) allows use of property TunnelInterface but the other command does not (New-AzLoadBalancerBackendAddressPool)

Adding the same parameter "TunnelInterface" to New-AzLoadBalancerBackendAddressPool and Set-AzLoadBalancerBackendAddressPool

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)
    • {Please put the link here}
  • the markdown help files have been regenerated using the commands listed here

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/azp run

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