Implement MySQL database adapter

## Description
Create MySQL database adapter following the same pattern as PostgreSQL implementation.

## Requirements
- [ ] Implement Database trait for MySQL
- [ ] Add MySQL-specific connection handling
- [ ] Support MySQL connection strings
- [ ] Handle MySQL-specific SQL syntax
- [ ] Add proper identifier quoting (backticks)
- [ ] Write comprehensive tests

## Acceptance Criteria
- MySQL adapter implements Database trait
- Proper connection management with mysql_async or sqlx
- MySQL-specific SQL generation
- Feature flag for MySQL support

## Files to create
- `beatrix_core/src/relational/db/mysql.rs`
- Update `beatrix_core/Cargo.toml` with MySQL dependencies

## Example Usage
```rust
use beatrix::relational::MySQL;

let db = MySQL::new("mysql://user:pass@localhost/db").await?;
let users = User::select().fetch_all(&db).await?;
```