Add SQL function to get backendId for temp table

[robins]

https://github.com/postgres/postgres/blob/master/src/backend/catalog/namespace.c#L3313

/*

• GetTempNamespaceBackendId - if the given namespace is a temporary-table
• namespace (either my own, or another backend's), return the BackendId
• that owns it. Temporary-toast-table namespaces are included, too.
• If it isn't a temp namespace, return InvalidBackendId.
  */
  int
  GetTempNamespaceBackendId(Oid namespaceId)
  {