Support MySQL as Metastore

### Is your feature request related to a problem? Please describe.

Currently, Polaris only supports PostgreSQL and H2 databases for its relational JDBC persistence layer. Many organizations have existing MySQL infrastructure and expertise, making MySQL support a valuable addition for wider adoption.

### Describe the solution you'd like

Add MySQL support to the relational-jdbc persistence module by:

1. Add MySQL to DatabaseType enum: Extend DatabaseType.java to include MySQL as a supported database type.
2. Create MySQL-specific schema files: Add mysql/ directory under persistence/relational-jdbc/src/main/resources/ with MySQL-compatible schema-v2.sql (v1 is not needed for new database support).
3. Handle MySQL-specific data types in schema:
- Use JSON instead of PostgreSQL's JSONB
- Use TEXT instead of large VARCHAR for column size limitations
4. Update build dependencies: Add MySQL JDBC driver to persistence/relational-jdbc/build.gradle.kts: implementation(libs.mysql.connector.java)
5. Modify QueryGenerator for schema handling: Update getFullyQualifiedTableName() method to handle MySQL's database naming (without schema prefix):
6. Extend existing database type handling in Model classes: The JSON handling logic already exists in ModelEntity.toMap() and ModelPolicyMappingRecord.toMap() - just need to add MySQL case to use plain string instead of PGobject.

### Describe alternatives you've considered

_No response_

### Additional context

MySQL was previously supported through the EclipseLink persistence implementation (see issue https://github.com/apache/polaris/issues/241 and PR https://github.com/apache/polaris/pull/247), but this support is now deprecated since EclipseLink was deprecated in favor of the relational-jdbc implementation. The relational-jdbc module, which is now the recommended production persistence backend, does not currently include MySQL support.

Start with DatabaseType.java, persistence/relational-jdbc/build.gradle.kts, QueryGenerator.getFullyQualifiedTableName(), and the ModelEntity.toMap() and ModelPolicyMappingRecord.toMap() methods. Compare the existing PostgreSQL schema with a new persistence/relational-jdbc/src/main/resources/mysql/schema-v2.sql, then run the relational-jdbc module checks to verify MySQL schema handling, driver setup, table naming, and JSON mapping.

Written by the indexing model from the issue text.