Change: Made DefaultUserSession.AuthenticateAsync overrideable so that (#2607)
Change: Made DefaultUserSession.AuthenticateAsync overrideable so that (#2607)

it will be easier to support user impersonation.
MienDev/IdentityServer4,MienDev/IdentityServer4,IdentityServer/IdentityServer4,IdentityServer/IdentityServer4,IdentityServer/IdentityServer4,MienDev/IdentityServer4,IdentityServer/IdentityServer4,MienDev/IdentityServer4