JWT auth support

[slvrtrn]

Summary

This PR adds support for JWT authentication (ClickHouse Cloud feature).

• Add JWT to the client configuration
• Allow to override the token

Checklist

• Unit and integration tests covering the common scenarios were added
• A human-readable description of the changes was provided to include in CHANGELOG
• For significant changes, documentation in https://github.com/ClickHouse/clickhouse-docs was updated with further explanations or tutorials

[slvrtrn]

As JWT expires over time, we need a way to override it in the client.

@genzgd, perhaps a set_access_token method on the client instance could be a good idea? An alternative is to add authorization override on all the public methods, which is probably too much and won't be too convenient to use. WDYT?

[genzgd]

set_access_token makes sense. If we wanted to get fancy we could make the access token constructor parameter take either a string or a function/hook that returns a string so the user could use their own implementation for refreshing?

[mzitnik]

LGTM

[slvrtrn]

I added the set_access_token method for starters. We can provide more exquisite support with internal timers as a follow-up.