Improvements to WebAdministration module

[nick-o]

Hey everyone,

this Pull Request includes two changes to the msWebAdministration module:

1. Extending the msWebAppPool resource to accept an optional [PSCredential]$Credentials parameter along with a [string]$IdentityType parameter that allows for the AppPool to be run under a specific user
2. Extending the msWebVirtualDirectory resource to accept a [PSCredential]$Credential parameter that allows to run a Virtual Directory as a specific user

The above functionality is required by at least two customers we are currently onboarding and has been in production for one existing customer off my fork for quite a while now.

Please merge at your earliest convenience.

Thanks
Nico