graph:Add stubs for education/classes endpoints (#5360)

* Renamed files for consistency reasons

err_school.go implements the full education interface not just schools.
ldap_school.go renamed to ldap_education_school.go for making it
consistent with ldap_education_user.go

* graph: Add stubs for education/classes endpoints

The acutal backend implementations are still empty.

services/graph/pkg/identity/ldap_education_class.go

@@ -0,0 +1,33 @@
+package identity
+
+import (
+	"context"
+	"net/url"
+
+	libregraph "github.com/owncloud/libre-graph-api-go"
+)
+
+// GetEducationClasses implements the EducationBackend interface for the LDAP backend.
+func (i *LDAP) GetEducationClasses(ctx context.Context, queryParam url.Values) ([]*libregraph.EducationClass, error) {
+	return nil, errNotImplemented
+}
+
+// CreateEducationClass implements the EducationBackend interface for the LDAP backend.
+func (i *LDAP) CreateEducationClass(ctx context.Context, class libregraph.EducationClass) (*libregraph.EducationClass, error) {
+	return nil, errNotImplemented
+}
+
+// GetEducationClass implements the EducationBackend interface for the LDAP backend.
+func (i *LDAP) GetEducationClass(ctx context.Context, namedOrID string, queryParam url.Values) (*libregraph.EducationClass, error) {
+	return nil, errNotImplemented
+}
+
+// DeleteEducationClass implements the EducationBackend interface for the LDAP backend.
+func (i *LDAP) DeleteEducationClass(ctx context.Context, nameOrID string) error {
+	return errNotImplemented
+}
+
+// GetEducationClassMembers implements the EducationBackend interface for the LDAP backend.
+func (i *LDAP) GetEducationClassMembers(ctx context.Context, nameOrID string) ([]*libregraph.EducationUser, error) {
+	return nil, errNotImplemented
+}