appgw: validate cert only if certificateFile is specified in set ssl cert command

[akshaysngupta]

Description

This PR majorly is to fix #9488

But as a part of this PR, we add a test in Network project using a KeyVault dataplane cmdlets. To make these cmdlets work, I am have made additional changes to Network Test Runner and KeyVault project. These changes help to mock the keyvault client in the Network project.

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

[VeryEarly]

Hi @akshaysngupta ,

Could you please also update the changelog.md?

[msJinLei]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).

[msJinLei]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).

[anton-evseev]

Hi @akshaysngupta, changes from network-march were merged into master. Please retarget your PR to master as well to make the release. Please also note that master was rebased due to credentials leaking into it so it's basically incompatible with network-march. My suggestion is to create a new branch from master and copy your changes there.

[anton-evseev]

@akshaysngupta please re-create PR using master as base

[RustyF]

Any idea what release this will be in?