Allow credentials override for a particular request

[slvrtrn]

Summary

Allows to override username/password for a particular request (query/command/exec/insert). Resolves #278

Checklist

• [x] Unit and integration tests covering the common scenarios were added
• [x] 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

[sonarcloud[bot]]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
89.5% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud