feat: implement TDigest for approx quantile

Adds a [TDigest] implementation providing approximate quantile
estimations of large inputs using a small amount of (bounded) memory.

A TDigest is most accurate near either "end" of the quantile range (that
is, 0.1, 0.9, 0.95, etc) due to the use of a scalaing function that
increases resolution at the tails. The paper claims single digit part
per million errors for q ≤ 0.001 or q ≥ 0.999 using 100 centroids, and
in practice I have found accuracy to be more than acceptable for an
apprixmate function across the entire quantile range.

The implementation is a modified copy of
https://github.com/MnO2/t-digest, itself a Rust port of [Facebook's C++
implementation]. Both Facebook's implementation, and Mn02's Rust port
are Apache 2.0 licensed.

[TDigest]: https://arxiv.org/abs/1902.04023
[Facebook's C++ implementation]: https://github.com/facebook/folly/blob/main/folly/stats/TDigest.h

datafusion/src/physical_plan/mod.rs

@@ -655,6 +655,7 @@ pub mod sort;
 pub mod sort_preserving_merge;
 pub mod stream;
 pub mod string_expressions;
+pub(crate) mod tdigest;
 pub mod type_coercion;
 pub mod udaf;
 pub mod udf;