Feature: Devstats - histogram of author buckets to commits

[jpmcb]

For a given repo, to surface the % of author commits for the given authors in a time range:

the API would need to support a histogram that captures:

{
  bucket: author_name,
  commits: #
}

This way, a client could derive a histogram chart (as pictured above) or tease apart the data to capture percentages of core authors, etc.

The commits could be a simple time_bucket on the timescale data in merged pull_request_github_events and the assocated push_github_events commits that actually end up landing on the main branch.

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Related: https://github.com/open-sauced/engineering/issues/37