Add default HTTP settings + readonly user switch

[slvrtrn]

Summary

See CHANGELOG. It is a breaking change for read-only users, but IMO necessary, as timeout issues when an LB terminates a long-running query connection are incredibly obscure, and this should at least cover AWS LB defaults.

It works similarly to how clickhouse-connect handles it (but currently does not normalize it based on the request timeout).

Checklist

• Unit and integration tests covering the common scenarios were added
• A human-readable description of the changes was provided to include in CHANGELOG
• Docs will be updated if we decide to merge this.

[mshustov]

in clickhouse, readonly has a bit different semantics: 0, 1, 2. let's align

[slvrtrn]

@mshustov, I updated the version, docs, and CHANGELOG, plus added an example, WDYT?