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

Adds the ApproxQuantile physical expression, plumbing & test cases.

The function signature is:

	approx_quantile(column, quantile)

Where column can be any numeric type (that can be cast to a float64) and
quantile is a float64 literal between 0 and 1.

Adds the approx_quantile() dataframe function, and exports it in the
prelude.

Adds bastilla wire encoding for approx_quantile.

Adding support for this required modifying the AggregateExprNode proto
message to support propigating multiple LogicalExprNode aggregate
arguments - all the existing aggregations take a single argument, so
this wasn't needed before.

This commit adds "repeated" to the expr field, which I believe is
backwards compatible as described here:

	https://developers.google.com/protocol-buffers/docs/proto3#updating

Specifically, adding "repeated" to an existing message field:

	"For ... message fields, optional is compatible with repeated"

No existing tests needed fixing, and a new roundtrip test is included
that covers the change to allow multiple expr.

Casts the calculated quantile value to the same type as the input data.

Ensures the quantile values is between 0 and 1, emitting a plan error if
not.