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

• 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

[mshustov]

considering that we require both username and password to be either defined or not, maybe we should put it under a single field

auth?: { username: string, password: string } | { token: string }

[sonarcloud]

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