Summary:
Point to a private form of tdigest code.
This is used for quantiles (coming soon).

Test Plan: N/A

Reviewers: michelle, oazizi, #engineering

Reviewed By: oazizi, #engineering

Differential Revision: https://phab.corp.pixielabs.ai/D439

GitOrigin-RevId: e4baf00e4e92d9cee449e583cf14414e00e911da