Get point-in-time TDigest snapshot from BasicQuantileStat
Get point-in-time TDigest snapshot from BasicQuantileStat

Summary:
This diff enables aggregation of point-in-time snapshots of the `TDigest`s that represents the state of `BasicQuantileStat`

We capture a `Snapshot` of `TDigest` and `SlidingWindow<TDigest>`.

The diff exposes `TDigest` and `SlidingWindow<TDigest>` to `QuantileStats`.

These objects are wrapped by the structs, `Snapshot` and `SnapshotEntry`, in `BasicQuantileStat` and `BasicQuantileStatMap`, respectively .

Reviewed By: yfeldblum

Differential Revision: D25170883

fbshipit-source-id: 0201a05cc4020851b652ed3a7b602d5b22aa63be

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