feat(api): buckets in experiments run sums

[inimaz]

Description

Allow the API to reply the experiment run sums in buckets instead of giving all runs. Like this the user can say max_points=200 and the API will aggregate them. This PR contains only the backend part. Frontend changes will come next

Related Issue

Related to #1232

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• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

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[davidberenstein1957]

Merged master into your branch: the conflicts were master's new auth context in runs.py and experiment_sum_by_run.py; the bucketed path now runs behind the same can_read_experiment check, and the two bucket tests pass auth_context. API unit tests pass; the integration tests (bucket SQL against Postgres, 403 without read access) still need a run. Please check before merging.