Add how to get cert and save it as pfx

[juntakata]

Some customers are asking Azure technical support how to download certificates on Key Vault as pfx files. I added few commands to do it.

Description

Some customers are asking Azure technical support how to download certificates on Key Vault as pfx files. I added few commands to do it.

Checklist

• [x ] 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

[adxsdkps]

Can one of the admins verify this patch?

[markcowl]

@RandalliLama @schaabs This looks good to me, but can you take a look?

[markcowl]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).

[markcowl]

Also, please add a Changelog entry for Az.KeyVault undre src\KeyVault\KeyVault\Changelog.md

[markcowl]

The help format won't allow two different code blocks - instead, please just make this a separate example. Also, we will need to add suppressions for the mock secrets in your example, as our credential scanner has flagged them.

[SpeedySquid]

Typo - 'SecredId' should be 'SecretId'