Feature Request—Consider providing ACL in ASP.NET Core Identity

To get some context about the request, see the issue https://github.com/aspnet/Identity/issues/1859

## Consider providing ACL in Identity
Well as in the title: consider providing ACL in ASP.NET Core though Identity.

### Some arguments
- ACL is not something too much complex for enterprise level app…
- What Identity provides today, **about group management**, is out to date (again see the issue [1859](https://github.com/aspnet/Identity/issues/1859))…
- Other frameworks oriented **enterprise level app/webapp**, provide, out of the box, ACL support. (See, Zend frameworks, Joomla, etc…)
- **With all due respect, In comparison, Identity looks like a back to the Past…(there is simply no ACL support at all)**
- Nothing prevents to suggest some improvements, **[or do not mention community involvement](https://www.asp.net/open-source)**…
- You should "write" [for customers, **not for simplicity sake**, nor without listening customers](https://github.com/aspnet/Identity/issues/1859#issuecomment-403067317)… And I used to think that ASP.NET Core was for writing enterprise level app, not to write simple blog CMS, **with simplistic group support**…
- it will not cost so much for you, to add ACL support.

### Design
It will be simple to integrate some ACL support to Identity, without breaking anything (simply by adding few line of codes, again, take a look at [1859](https://github.com/aspnet/Identity/issues/1859)).

For the sake of demonstration:
- Add an Acl table like `IdentityAcl`
- Add a many-to-many relationship table between `IdentityRoles` and `IdentityAcl`;
- Add to `ClaimsIdentity` and `ClaimsPrincipal` … the adequat properties (to avoid having recourse to `Scope` and `IdentityDbContext` for `AuthorizationHandler`)…
- Good doc…

### Some thoughts
We do not ask you to write for us our apps, but to provide us with a good, up to date, and real world use framework (**like other frameworks in the market**).