Feature: S2S user delegation in Hightrust app scenario

[tmeckel]

Q	A
Bug fix?	no
New feature?	yes
New sample?	no
Related issues?	n/a

What's in this Pull Request?

This PR contains an enhancement whith which it's possible to create an App Delegation token in an Hightrust configuration to act as specific user (behalf-of) within a SharePoint connection. Up to now this scenario was only possible with an WindowsIdentity object instance available, which is not the case if the addin is deployed in an environment which isn't connected to an Active Directory environment (e.g. Azue WebApp) and is employing other authentication themes like JWT Bearer or SAML tokens.

This PR now provides new methods on Core/OfficeDevPnP.Core/AuthenticationManager.cs which allows to pass the SID or the UPN for an user for creating an impersonation (delegation) token. The SID is just as unavailable in the above scenario as the WindowsIdentity object instance but the UPN typically is, because the UPN is normally used in JWT or SAML token to identify an incoming user.

With this extension it is possible to host a SharePoint Add in a "domain-less" environment, like as an Azure WebApp, utilize modern authentication methods and still perform impersonation on a SharePoint system in an Active Directory domain.

[jansenbe]

Thanks @tmeckel. Just merged this one, did revert the public method rename you did to avoid breaking folks that were already using this.

[tmeckel]

Thanks @tmeckel. Just merged this one, did revert the public method rename you did to avoid breaking folks that were already using this.

@jansenbe Thanks for merging my PR and spotting the breaking change and reverting it!