Make Roles optional at the Microsoft.Extensions.Identity layer

[HaoK]

High level idea is to enable something akin to:

Related to #651

I think this should be safe to do in 2.1, but we might want to consider doing something like this in RTM (as this is really finishing what we started in 651)

services.AddIdentityCore<TUser>() // UserManager related servives
   .AddRoles<TRole>() // RoleManager related services
   .AddEntityFrameworkStores<TContext>(storeOptions => { }) // All stores needed for all managers, optional schema tweaking
   .AddSignInManager() // SignInManager related stuff

Would be equivalent to today's (which we would still support):

services.AddIdentity<TUser, TRole>().AddEntityFrameworkStores<TContext>()

[divega]

@HaoK do you always need to configure in storeOptions in the call to AddEntityFrameworkStores to indicate whether the schema for roles needs to be included or can that be inferred automatically by the fact that you called AddRoles?

[HaoK]

Its an optional thing, I just was showing it to make it obvious that was where you will can opt into/out of things.

AddEntityFrameworkStores() would do something by default appropriately...

[HaoK]

But its likely there will be ways to mismatch things, like calling AddRoles() and then doing storeOptions.SupportsRoles = false... We won't be able to block all of those, but it should be easy to validate

[HaoK]

Basically misconfiguring storeOptions will cause manager apis to throw if they end up calling into store functionality that they disable

[HaoK]

Also related to #581

[HaoK]

After chatting with @ajcvickers today we agreed that this is something important to do in 2.0, some nice sideffects that fell out of the refactoring/exploring this. we can nuke all of the entities on TUser/TRole except for TKey... That cleans things up quite a bit.