Identityserver4.admin: Add flag for exluding Asp.Net Core Identity from dotnet new template

[skoruba]

Currently dotnet new template contains following params:

dotnet new skoruba.is4admin 
--name MyProject 
--title MyProject 
--adminrole MyRole 
--adminclientid MyClientId

It's necesarry to add the flag --exludeAspNetIdentity for exluding Asp.Net Core Identity from template.

[fudgeboy2000]

This project is awesome - many thanks for all your efforts! We have an existing IS4 implementation for SSO purposes which doesn't use .NET Identity, so this flag would be the magic bullet for adding Admin to our solution as things stand.