Add ChallengeProviderTimeout type to acme package

This type allows for implementing DNS ChallengeProviders that require
an unsually long timeout when checking for record propagation.

acme/dns_challenge.go

@@ -69,7 +69,15 @@ func (s *dnsChallenge) Solve(chlng challenge, domain string) error {
 
 	logf("[INFO][%s] Checking DNS record propagation...", domain)
 
-	err = WaitFor(60*time.Second, 2*time.Second, func() (bool, error) {
+	var timeout, interval time.Duration
+	switch provider := s.provider.(type) {
+	case ChallengeProviderTimeout:
+		timeout, interval = provider.Timeout()
+	default:
+		timeout, interval = 60*time.Second, 2*time.Second
+	}
+
+	err = WaitFor(timeout, interval, func() (bool, error) {
 		return preCheckDNS(fqdn, value)
 	})
 	if err != nil {

acme/provider.go

@@ -1,5 +1,7 @@
 package acme
 
+import "time"
+
 // ChallengeProvider enables implementing a custom challenge
 // provider. Present presents the solution to a challenge available to
 // be solved. CleanUp will be called by the challenge if Present ends
@@ -8,3 +10,19 @@ type ChallengeProvider interface {
 	Present(domain, token, keyAuth string) error
 	CleanUp(domain, token, keyAuth string) error
 }
+
+// ChallengeProviderTimeout allows for implementing a
+// ChallengeProvider where an unusually long timeout is required when
+// waiting for an ACME challenge to be satisfied, such as when
+// checking for DNS record progagation. If an implementor of a
+// ChallengeProvider provides a Timeout method, then the return values
+// of the Timeout method will be used when appropriate by the acme
+// package. The interval value is the time between checks.
+//
+// The default values used for timeout and interval are 60 seconds and
+// 2 seconds respectively. These are used when no Timeout method is
+// defined for the ChallengeProvider.
+type ChallengeProviderTimeout interface {
+	ChallengeProvider
+	Timeout() (timeout, interval time.Duration)
+}