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

Types of changes

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

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