New extension design

[PaulHigin]

This is the sum total of multiple commits for Secrets Management, using the new C# abstract class vault extension mechanism. Vault extension is based on PowerShell modules. One method is a PowerShell module with a RequiredAssembly having the C# class type implementation, and the other is a script module that implements required extension methods as script functions.

[SteveL-MSFT]

Now it says Modules/Microsoft.PowerShell.SecretsManagement/TestExtensionModules/AKVaultScript/AKVaultScript.psm1 is a binary file