feat(query): make product name configurable

[sundy-li]

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Summary

Add a query-level product_name config with the default value Databend Query, and use it when building the non-MySQL server version string returned by version().

This allows OEM deployments to configure a user-visible product name at runtime without rebuilding the binary. MySQL compatibility version strings remain unchanged.

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - Explain why

Validation:

cargo fmt --check
cargo test -p databend-query --test it configs -- --nocapture

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
• Other (please describe):

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This change is