Add a helper extension for setting up authentication with Azure AD B2C

[danroth27]

We should provide some extensions that simplify setting up authentication with Azure AD B2C for both web apps and web APIs. Here is what the templates current do:

https://github.com/dotnet/templating/tree/rel/vs2017/3-Preview2/template_feed/Microsoft.DotNet.Web.ProjectTemplates.2.0/content/StarterWeb-CSharp/Extensions/AzureAdB2C

https://github.com/dotnet/templating/tree/rel/vs2017/3-Preview2/template_feed/Microsoft.DotNet.Web.ProjectTemplates.2.0/content/WebApi-CSharp/Extensions/AzureAdB2C

See also #1213 and #1212

[danroth27]

We should fix #1220 as part of this

[danroth27]

@ajcvickers @Eilon @muratg

@HaoK I believe this should be assigned to you, correct? Are you going to be able to get to this by EOW?