Syncing: move global ID generation logic to an overridable method

Also make all resource syncing-related listener closures static.

Also correct return type for getGlobalIdentifierKey to string|int.
(We intentionally do not support returning null like many other
"get x key" methods would since such a case might break resource
syncing logic. This is also why we use inline getAttribute() in the
creating listener instead of calling the method.)

tests/ResourceSyncingTest.php

@@ -958,12 +958,14 @@ test('pivot record is automatically deleted with the tenant resource', function(
 
     $syncMaster->tenants()->attach($tenant);
 
+    expect(DB::select("SELECT * FROM tenant_users WHERE tenant_id = ?", [$tenant->id]))->toHaveCount(1);
+
     $tenant->run(function () {
         TenantUser::firstWhere('global_id', 'cascade_user')->delete();
     });
 
     // Deleting tenant resource deletes its pivot record
-    expect(DB::select("SELECT * FROM tenant_users WHERE tenant_id = ?", [$tenant->getTenantKey()]))->toHaveCount(0);
+    expect(DB::select("SELECT * FROM tenant_users WHERE tenant_id = ?", [$tenant->id]))->toHaveCount(0);
 });
 
 test('DeleteAllTenantMappings handles incorrect configuration correctly', function() {