chore: create type for unique role names (#13506)

* chore: create type for unique role names

Using `string` was confusing when something should be combined with
org context, and when not to. Naming this new name, "RoleIdentifier"

coderd/database/dbfake/dbfake.go

@@ -26,7 +26,7 @@ import (
 
 var ownerCtx = dbauthz.As(context.Background(), rbac.Subject{
 	ID:     "owner",
-	Roles:  rbac.Roles(must(rbac.RoleNames{rbac.RoleOwner()}.Expand())),
+	Roles:  rbac.Roles(must(rbac.RoleIdentifiers{rbac.RoleOwner()}.Expand())),
 	Groups: []string{},
 	Scope:  rbac.ExpandableScope(rbac.ScopeAll),
 })