Allow to specify tls certificate path though DSN

### Feature request description

For now the only way to add TLS validation is though code with [RegisterTLSConfig](https://godoc.org/github.com/go-sql-driver/mysql#RegisterTLSConfig). With providing certificate path though DSN ([like pq is doing it](https://godoc.org/github.com/lib/pq#hdr-Connection_String_Parameters)) all existing applications will receive TLS certificate validation out of the box without code changes. It will also help the applications that supports both pq and mysql as this will eliminate mysql-specific initialisation code before `sql.Open()` call.