feat: add MySQL metadata provider support

[shefeek-jinnah]

Summary

• Add MySQL as a supported metadata provider backend for DuckLake catalogs
• Implement MySqlMetadataProvider following the existing PostgreSQL pattern
• Enable managed MySQL service deployments (AWS RDS, Azure MySQL, Google Cloud SQL, etc.)

#33

Usage

use datafusion_ducklake::{DuckLakeCatalog, MySqlMetadataProvider};

let provider = MySqlMetadataProvider::new("mysql://user:pass@host:3306/db").await?;
let catalog = DuckLakeCatalog::new(provider)?;
ctx.register_catalog("ducklake", Arc::new(catalog));