T-Digest (Re)Construction

[nbehnam]

I'm working on some integration with the Java implementation and found that it could be helpful to have more verbose construction such that I can recreate a t-digest given the component parts (compression / centroids (and min/max?)). This is basically already done in internally in the fromBytes code but isn't accessible via a public method.

It could also aid in re-construction/ergonomics to have other serialization options such as JSON to help with cross-language support, in the case I create my t-digest via a Spark UDAF but want to consume it in my python application without re-implementing the bytes encoding.