Add support for custom HTTP/HTTPS agent in createClient configuration

[yosefrev]

Summary

A short description of the changes with a link to an open issue.

Checklist

Delete items not relevant to your PR:

• [ ] 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

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