Expose PerSubscriberAggregate

[jc-harrison]

Expose PerSubscriberAggregate through the API, and add a corresponding flowclient query spec. This query should be exposed as a subscriber metric that can be used in 'joined_spatial_aggregate' and 'histogram_aggregation'.

The 'subscriber_query' parameter should accept any subscriber metric query, or a union of subscriber metric queries. Given that Union is not exposed through the API, and a general Union would not be suitable here (only a Union of the accepted subscriber metric queries), I'd suggest we could allow a list of subscriber metric queries as input for this parameter (similar to modal_location taking a list of daily locations), and construct the union internally.

I'd also suggest that we don't expose the 'agg_column' parameter, and instead hard-code it to 'value'.