Add ZSON extension to /contrib/

Hi hackers,

Back in 2016 while being at PostgresPro I developed the ZSON extension [1].
The extension introduces the new ZSON type, which is 100% compatible with
JSONB but uses a shared dictionary of strings most frequently used in given
JSONB documents for compression. These strings are replaced with integer
IDs. Afterward, PGLZ (and now LZ4) applies if the document is large enough
by common PostgreSQL logic. Under certain conditions (many large
documents), this saves disk space, memory and increases the overall
performance. More details can be found in README on GitHub.

The extension was accepted warmly and instantaneously I got several
requests to submit it to /contrib/ so people using Amazon RDS and similar
services could enjoy it too. Back then I was not sure if the extension is
mature enough and if it lacks any additional features required to solve the
real-world problems of the users. Time showed, however, that people are
happy with the extension as it is. There were several minor issues
discovered, but they were fixed back in 2017. The extension never
experienced any compatibility problems with the next major release of
PostgreSQL.

So my question is if the community may consider adding ZSON to /contrib/.
If this is the case I will add this thread to the nearest CF and submit a
corresponding patch.

[1]: https://github.com/postgrespro/zson

--
Best regards,
Aleksander Alekseev
Open-Source PostgreSQL Contributor at Timescale