server, ldap: add support for Subschema requests (#1071)

Add a subschema entry to the rootDSE, which shows all attributes and
objectclasses present on the LLDAP server, which is needed for some
applications that need to index the LDAP server. The current
implementation's goal is to have a bare minimum working subschema which
follows the LDAP RFC. It also updates the GraphQL interface to follow
the changes that have been made in actually separating out
objectclasses, instead of having them as an attribute.

Co-authored-by: nitnelave <valentin@tolmer.fr>

schema.graphql

@@ -80,6 +80,11 @@ input CreateUserInput {
   "Attributes." attributes: [AttributeValueInput!]
 }
 
+type ObjectClassInfo {
+  objectClass: String!
+  isHardcoded: Boolean!
+}
+
 type AttributeSchema {
   name: String!
   attributeType: AttributeType!
@@ -186,6 +191,7 @@ enum AttributeType {
 type AttributeList {
   attributes: [AttributeSchema!]!
   extraLdapObjectClasses: [String!]!
+  ldapObjectClasses: [ObjectClassInfo!]!
 }
 
 type Success {