[KeyVault] Support getting plain text secret with `-AsPlainText`

[isra-fel]

Description

Adding an optional parameter to Get-AzKeyVaultSecret. When specified, outputs the plain text secret.

Design choices

• Should we return the plain text secret only, or return a PSKeyVaultSecret object with an extra property representing the plain text secret?
  • Per below sggestions, we'll just return the secret text.
• Any other cmdlets also need -AsPlainText?
  • Not this time.

Expectation

It should work in

• PowerShell 7.0, 7.1
• PowerShell core
• Windows PowerShell 5.1
• ConstrainedLanguage mode
• Linux
• MacOS

Cost

s

[dcaro]

cc @TylerLeonhardt

[SteveL-MSFT]

When AzKeyVault is used with https://www.powershellgallery.com/packages/Microsoft.PowerShell.SecretManagement module, you can use Get-Secret -AsPlainText. Also, with PowerShell 7, you can use ConvertFrom-SecureString -AsPlainText instead of calling the .NET APIs.

[TylerLeonhardt]

@SteveL-MSFT problem is on SAWs where:

• No SecretManagement
• No PS7

I think Azure PowerShell should align with the behavior in SecretManagement and just return the string.