OxQL: support using 'group_by' with N-dimensional tables

[wfchandler]

Currently the group_by operator in OxQL requires that the table schema be one-dimensional. For queries that span multiple similar tables, e.g. virtual_disk:failed_reads, virtual_disk:failed_writes, being able to group the results would be convenient.

Example query:

{
    get virtual_disk:failed_reads;
    get virtual_disk:failed_writes;
    get virtual_disk:failed_flushes
}
    | filter timestamp > @now() - 1d
    | align mean_within(5m)
    | join
    | group_by [attached_instance_id]