Export ConnectToProxy() for some backoff stage

[liut]

• Export ConnectToProxy() can support backoff for keepalive, like github.com/cenkalti/backoff, code example:

import "time"
import "github.com/cenkalti/backoff"

func expBackoff() backoff.BackOff {
	b := backoff.NewExponentialBackOff()
	b.InitialInterval = 500 * time.Millisecond
	b.Multiplier = 1.5
	b.MaxInterval = 60 * time.Second
	b.MaxElapsedTime = 0 // never stop

	return b
}

// Retry ...
func Retry(o backoff.Operation) error {
	return backoff.RetryNotify(o, expBackoff(), func(err error, d time.Duration) {
		log.Print("retry", "err", err, "d", d)
	})
}

main() {
	Retry(func() error {
		return remotedialer.ConnectToProxy(context.Background(), uri, headers, 
			func(string, string) bool { return true }, nil, nil)
	})
}

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