graph: Allow to disable the default role assignment on user creation

Introduces a switch ('GRAPH_ASSIGN_DEFAULT_USER_ROLE') to allow to disable the assignment of the default role "User" to newly created users.
This will be used for setups where the role-assignments are populated either manually or during first login (e.g. from OIDC claims)

services/graph/pkg/config/defaults/defaultconfig.go

@@ -40,6 +40,7 @@ func DefaultConfig() *config.Config {
 		API: config.API{
 			GroupMembersPatchLimit: 20,
 			UsernameMatch:          "default",
+			AssignDefaultUserRole:  true,
 		},
 		Reva: shared.DefaultRevaConfig(),
 		Spaces: config.Spaces{