Support non-standard transaction isolation levels

Some databases support non-standard transaction isolation levels. For example, MS SQL Server has a SNAPSHOT isolation level. Agroal currently can't support this via the transaction isolation level config option because TransactionIsolation is an enum, so it can't have custom values. It would be nice if this were supported. WildFly's "old" datasources subsystem already supports non-standard isolation levels, so it would be nice if the Agroal subsystem had parity.

There is a workaround, at least on MS SQL Server. initialSql (new-connection-sql in WildFly) can be used to execute "SET TRANSACTION ISOLATION LEVEL SNAPSHOT" when the connection is created.