Prepared statement can't use MySqlDbType.JSON

[bgrainger]

Reported for Connector/NET: https://bugs.mysql.com/bug.php?id=95984

MySQL Server disallows MYSQL_TYPE_JSON as a prepared statement column type: https://github.com/mysql/mysql-server/blob/124c7ab1d6f914637521fd4463a993aa73403513/sql/sql_prepare.cc#L636-L639

However, MySqlDbType.VarChar or MySqlDbType.Binary will be accepted (and parsed/validated as JSON). This code needs to be fixed to send JSON as String:

MySqlConnector/src/MySqlConnector/Core/TypeMapper.cs

Lines 377 to 378 in edba329

	case MySqlDbType.JSON:
	columnType = ColumnType.Json; // TODO: test

[bgrainger]

Fixed in 0.57.0.