[Feature Request] Allow unwrapping otelConn to get underlying driver conn

[mgabeler-lee-6rs]

I have an app that uses *sql.Conn.Raw(...) in places, and I'd like to add otelsql instrumentation to the app. However the places that use Raw() get stuck because they get a *otelsql.otelConn instead of the underlying driver connection, and can't unwrap it (without reflection) to get the real one.

It would be nice if there was a method such as Unwrap() driver.Conn which made this possible without reflection