Specify a custom dial function per config (go-sql-driver#1527)

Specify a custom dial function per config instead of using
RegisterDialContext.

connector.go

@@ -87,20 +87,25 @@ func (c *connector) Connect(ctx context.Context) (driver.Conn, error) {
 	mc.parseTime = mc.cfg.ParseTime
 
 	// Connect to Server
-	dialsLock.RLock()
-	dial, ok := dials[mc.cfg.Net]
-	dialsLock.RUnlock()
-	if ok {
-		dctx := ctx
-		if mc.cfg.Timeout > 0 {
-			var cancel context.CancelFunc
-			dctx, cancel = context.WithTimeout(ctx, c.cfg.Timeout)
-			defer cancel()
-		}
-		mc.netConn, err = dial(dctx, mc.cfg.Addr)
+	dctx := ctx
+	if mc.cfg.Timeout > 0 {
+		var cancel context.CancelFunc
+		dctx, cancel = context.WithTimeout(ctx, c.cfg.Timeout)
+		defer cancel()
+	}
+
+	if c.cfg.DialFunc != nil {
+		mc.netConn, err = c.cfg.DialFunc(dctx, mc.cfg.Net, mc.cfg.Addr)
 	} else {
-		nd := net.Dialer{Timeout: mc.cfg.Timeout}
-		mc.netConn, err = nd.DialContext(ctx, mc.cfg.Net, mc.cfg.Addr)
+		dialsLock.RLock()
+		dial, ok := dials[mc.cfg.Net]
+		dialsLock.RUnlock()
+		if ok {
+			mc.netConn, err = dial(dctx, mc.cfg.Addr)
+		} else {
+			nd := net.Dialer{}
+			mc.netConn, err = nd.DialContext(dctx, mc.cfg.Net, mc.cfg.Addr)
+		}
 	}
 	if err != nil {
 		return nil, err

dsn.go

@@ -55,6 +55,8 @@ type Config struct {
 	ReadTimeout          time.Duration     // I/O read timeout
 	WriteTimeout         time.Duration     // I/O write timeout
 	Logger               Logger            // Logger
+	// DialFunc specifies the dial function for creating connections
+	DialFunc func(ctx context.Context, network, addr string) (net.Conn, error)
 
 	// boolean fields