feat: add aggregation_units to metric and datasource

[mikewilli]

Step 1 towards supporting profile management feature.

• adds support for aggregation_units to metrics and data_sources, and defaults both to what is currently supported, client_id only
• ensures that the data_source supports (at least) all aggregations that the metric supports

[scholtzan]

I wonder if calling it group_by would be a better name?

[mikewilli]

To me group_by implies that we would group by the configured value(s). In this case, it's only meant to indicate which fields are supported as a group_by. Might still be fine / better than aggregation_units, but just concerned that people will think we are grouping by everything in the list. group_by_support maybe?

I'm definitely open to a different name though, and @danielkberry and I discussed it a bit already (analysis_units was another one we discussed).

[danielkberry]

Looks great! A few small suggestions and questions. I'm excited to incorporate this into my mozanalysis PR.

[danielkberry]

There's just one open question, then I can re-review.

[danielkberry]

Looks like all the changes are made, thanks! LGTM