[#4393] added Planning Center OAuth2 provider

Co-authored-by: alxjsn <alxjsn@sameorigin.org>

tools/auth/auth_test.go

@@ -234,4 +234,13 @@ func TestNewProviderByName(t *testing.T) {
 	if _, ok := p.(*auth.Bitbucket); !ok {
 		t.Error("Expected to be instance of *auth.Bitbucket")
 	}
+
+	// planningcenter
+	p, err = auth.NewProviderByName(auth.NamePlanningcenter)
+	if err != nil {
+		t.Errorf("Expected nil, got error %v", err)
+	}
+	if _, ok := p.(*auth.Planningcenter); !ok {
+		t.Error("Expected to be instance of *auth.Planningcenter")
+	}
 }