feat: JWT auth, minimal ClickHouse Cloud CI

[slvrtrn]

Summary

JWT auth is a new ClickHouse Cloud feature, introduced in January 2025.

• Added Client::with_access_token, slightly reworking the internal auth implementation.
• Client::with_user and Client::with_password will panic if called after Client::with_access_token (and vice versa).
• insert and query will use appropriate headers based on the authentication method.
• Added a minimal ClickHouse Cloud setup for CI JWT tests in GHA

Closes #183

Checklist

• Unit tests
• Integration tests
• A human-readable description of the changes was provided so that we can include it in CHANGELOG later