Update custom granularity spec

[courtneyholcomb]

Description

Rename custom_granularity_columns -> custom_granularities and add column_name property. This matches the newly-approved spec that will be added to core. This would be a breaking change, but since this isn't used anywhere yet, nothing will break.

Checklist

• [x] I have read the contributing guide and understand what's expected of me
• [x] I have signed the CLA
• [x] This PR includes tests, or tests are not required/relevant for this PR
• [ ] I have run changie new to create a changelog entry