Set charset to utf8mb4 for remote MySQL interactions

Why it matters

This feature addresses issues related to character encoding mismatches by ensuring that ClickHouse communicates with remote MySQL servers using the utf8mb4 charset. This improves compatibility and correctness when reading from or writing to MySQL databases that use extended UTF-8 characters.

How to use it

The charset is automatically set to utf8mb4 for all interactions with remote MySQL servers; no additional user configuration is required.