Support more compression methods for columnar (e.g. lz4hc)

[Columnar storage](https://github.com/citusdata/citus/tree/main/src/backend/columnar) page lists a lot of format limitations, including

> No support for foreign keys, unique constraints, or exclusion constraints

But Citus updates [post](https://www.citusdata.com/updates/v10-2/) claims:

> With the index support on columnar tables, Citus 10.2 supports the following constraints on columnar tables: primary keys, unique constraints, and exclude constraints.

This was a deciding factor for me and I almost declined Citus option when making a decision. If it's indeed supported, can someone fix the readme inaccuracy, please.