v7.0.0 of conduit_mysql

MySQL / MariaDB ORM backend for conduit. Schema management, raw execute path, and dialect-agnostic ORM newQuery (lifted from the postgresql package). Changelog excerpt: > Note: First publish to pub.dev. Requires Dart SDK >=3.12.0. - FEAT: add conduit_mysql backend + QueryPredicate AST migration (#267). - FEAT: multi-backend test harness + connection-string dispatch (#268). - REFACTOR: lift query builders to dialect-agnostic core; wire SQLite + MySQL newQuery (#275).