Clickhouse Backend

[EzePze]

Adds support for a ClickHouse backend. Based on the Postgres backend but using async ClickHouse drivers. Requested by #516

• [x] - Tested: Demo file created and example tables provided -- almost one-to-one with the Postgres demo
• [x] - Changelog updated
• [x] - Tests run and pass
• [x] - Flake8 run and all errors/warnings resolved
• [ ] - Contributors file updated (optional)

[EzePze]

Tried to go that route initially but there's enough small changes between the two drivers that mean almost everything has to be rewritten anyway (e.g. formatting every new row as tuples instead of strings) -- though could def be worthwhile making a parent class for SQL-like DBs that share the same methods.

[bmoscon]

old PR, closing