Return error in driver.BeginTx if options aren't used/required or incorrect.

[SimonRichardson]

According to the docs https://pkg.go.dev/database/sql#IsolationLevel

Various isolation levels that drivers may support in BeginTx. If a driver does not support a given isolation level an error may be returned

It would be good if the code return an error if any of the options are not supported. Looking at the driver code, we don't verify the options, but the documentation states that we may return an error. For consistency, it would be ideal to validate those options, so if new changes do arrive then code can be tested when they become valid.

https://github.com/canonical/go-dqlite/blob/f54a1528102399f99af4c1a8ebdbe14db2c68fda/driver/driver.go#L482C16-L493