Feedback suggestions 1

[PaulHigin]

Some various feedback suggestions from implementers:

1. Add parameter completion for -Vault parameter.
2. Ensure 'BuilitInLocalVault' default vault is consistent over all cmdlets.
3. Replace Get-SecretVault 'ImplementingType' property display with 'VaultType'.
4. Provide better information for when extension vault modules fail to load.
5. Update AzKeyVault test extensions to explicitly test for needed Az modules and provide better error message.
6. Fix ReadMe.md to change Add-Secret to Set-Secret.