Use a custom dialer to create connection with db

[XiaoYiXiaoYang]

Is your feature request related to a problem? Please describe. when i use "github.com/lib/pq" as sql driver to connect to postgressql i can use func DialOpen(d Dialer, dsn string) (_ driver.Conn, err error) method to connect to db with my custom dialer

and i suggest "github.com/denisenkom/go-mssqldb" need a same one

Describe the solution you'd like add a method like func DialOpen(d Dialer, dsn string) (_ driver.Conn, err error) before create connection

Describe alternatives you've considered

Additional context "github.com/go-sql-driver/mysql" have a similar method to use my own custom dialer， it is func RegisterDial(network string, dial DialFunc)

[niudaii]

Please see #665

type ProxyDialer struct {
    dialer proxy.Dialer
}

func (d ProxyDialer) DialContext(ctx context.Context, network string, addr string) (net.Conn, error) {
    return d.dialer.Dial(network, addr)
}

[XiaoYiXiaoYang]

thanks~~, it works for me good.