Add approx_quantile support

[domodwyer]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
I would like to efficiently aggregate (approximate) quantile values from a column of data - "show me the 99th percentile of the latency column in the requests table"

Describe the solution you'd like
Implement TDigest (or similar algorithm) to provide relatively cheap quantile values/estimations.

Describe alternatives you've considered
I've had a look at some other DBs:

• duckdb - tdigest & reservoir sampling
• timescaledb - tdigest & uddsketch
• snowflake - several options, including tdigest for cheap approximations
• presto - qdigest
• influxdb - tdigest

For approximate results, tdigest seems popular, though the uddsketch paper is relatively new and also interesting.

Additional context
Tdigest provides quantile estimatations, I imagine it would expose an approx_quantile(column, quantile) aggregation keeping with the naming of the approx_distinct() aggregation.

Example:

SELECT approx_quantile(latency, 0.99) AS p99 FROM requests;