Add repeatable SQL migration for function

`subjects_is_unique_type_identifier`

metasvc-server/backend-jdbi/src/main/resources/io/github/dbmdz/metadata/server/backend/impl/database/migration/R__func_subjects_is_unique_type_identifier.sql

@@ -0,0 +1,36 @@
+CREATE OR REPLACE FUNCTION public.subjects_is_unique_type_identifier(puuid uuid, ptype character varying, ids dbidentifier[], plabel jsonb)
+RETURNS boolean
+STABLE LANGUAGE plpgsql
+AS $function$
+/**
+ * Ensures uniqueness of `type` and single identifier across table `subjects`.
+ *
+ * If there are not any identifiers then `type` and `label` must be unique.
+ * It is intended for check constraints and
+ * returns `false` on violation.
+ */
+declare
+	db_ident record; --dbidentifier actually but makes trouble in backups
+	tuple_exists boolean;
+begin
+	if ids is null or cardinality(ids) = 0 then
+		execute 'select exists(select 1 from public.subjects where $1 = type and $2 = label and $3 <> uuid)'
+		  into tuple_exists
+		  using ptype, plabel, puuid;
+		if tuple_exists then
+			return false;
+		end if;
+	else
+		foreach db_ident in array ids loop
+			execute 'select exists(select 1 from public.subjects where $1 = type and $2 = any (identifiers) and $3 <> uuid)'
+			  into tuple_exists
+			  using ptype, db_ident, puuid;
+			if tuple_exists then
+				return false;
+			end if;
+		end loop;
+	end if;
+	return true;
+end;
+$function$;
+