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

[Copilot]

Pull Request Overview

This PR introduces JWT authentication via a new method on Client along with enhanced integration tests for ClickHouse Cloud and related CI configuration updates.

• Adds Client::with_access_token and updates authentication handling (credentials vs. JWT)
• Implements integration tests for JWT authentication and adjusts database creation to include a timestamp
• Refactors header handling and query/insert authentication while updating CI workflows and changelog

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/it/main.rs	Adds integration test documentation and environment-specific helpers
tests/it/cloud_jwt.rs	Introduces JWT-specific integration tests
src/watch.rs	Updates documentation for the fetch method in Watch
src/query.rs	Refactors query header injection by delegating to with_authentication
src/lib.rs	Implements Authentication enum and updates Client methods accordingly
src/insert.rs	Uses with_authentication for header injection in insert operations
src/headers.rs	Adds with_authentication helper for building request headers
CHANGELOG.md	Documents the new JWT authentication feature
.github/workflows/ci.yml	Revises CI configuration for supporting ClickHouse Cloud integration tests