Extension for sketch-based statistics

Goal : Provide an extension library to compute approximate statistics on streams.

Interest : Probabilistic data structures can create an approximation (sketch) of the current state of a stream without storing every element but rather processing each observation quickly to summarize its current state and find useful statistical insights.

Implementation is here : https://github.com/ArnaudFnr/beam/tree/sketching/sdks/java/extensions/sketching

More info : https://docs.google.com/document/d/1Xy6g5RPBYX_HadpIr_2WrUeusiwL0Jo2ACI5PEOP1kc/edit

Imported from Jira [BEAM-2728](https://issues.apache.org/jira/browse/BEAM-2728). Original Jira may contain additional context.
Reported by: arnaudfnr.

Review the proposed implementation in sdks/java/extensions/sketching and the linked design document first. Determine the extension’s intended approximate stream statistics and integration requirements from those materials. Done means the sketching extension is implemented and provides the described approximate statistics for streams.

Written by the indexing model from the issue text.