Provide more detailed information in certificate events (#150)

* Provide more detailed information in certificate events

The `cert_*` events were only providing the domain. This was pretty limited, because not much can be done with it without also knowing the issuer, since that affects where the cert is stored.

Providing both the `issuerKey` and `certKey` makes it possible for the event listener to call `StorageKeys.SiteCert(issuerKey, certKey)` to get the actual location of the cert (and similarly the private key). Particularly useful in situations where the user wants to copy the cert/key to some location for another app to use the cert that is being managed.

A similar change could probably also be done for `cached_managed_cert` and `cached_unmanaged_cert` events to provide more detail, because `cert.Names` string slice doesn't seem that useful, can't do much with that alone. But I'm not sure I understand the usefulness of those events, so meh. Caching seems like pretty much an internal implementation detail of certmagic.

* Adjust struct and field naming

* Mark CertificateEventData as experimental

config.go

@@ -573,7 +573,11 @@ func (cfg *Config) obtainCert(ctx context.Context, name string, interactive bool
 			return fmt.Errorf("[%s] Obtain: saving assets: %v", name, err)
 		}
 
-		cfg.emit("cert_obtained", name)
+		cfg.emit("cert_obtained", CertificateEventData{
+			Name:       name,
+			IssuerKey:  issuerUsed.IssuerKey(),
+			StorageKey: certRes.NamesKey(),
+		})
 
 		if log != nil {
 			log.Info("certificate obtained successfully", zap.String("identifier", name))
@@ -789,7 +793,11 @@ func (cfg *Config) renewCert(ctx context.Context, name string, force, interactiv
 			return fmt.Errorf("[%s] Renew: saving assets: %v", name, err)
 		}
 
-		cfg.emit("cert_renewed", name)
+		cfg.emit("cert_renewed", CertificateEventData{
+			Name:       name,
+			IssuerKey:  issuerUsed.IssuerKey(),
+			StorageKey: certRes.NamesKey(),
+		})
 
 		if log != nil {
 			log.Info("certificate renewed successfully", zap.String("identifier", name))
@@ -868,7 +876,11 @@ func (cfg *Config) RevokeCert(ctx context.Context, domain string, reason int, in
 			return fmt.Errorf("issuer %d (%s): %v", i, issuerKey, err)
 		}
 
-		cfg.emit("cert_revoked", domain)
+		cfg.emit("cert_revoked", CertificateEventData{
+			Name:       domain,
+			IssuerKey:  issuerKey,
+			StorageKey: certRes.NamesKey(),
+		})
 
 		err = cfg.deleteSiteAssets(ctx, issuerKey, domain)
 		if err != nil {
@@ -1093,6 +1105,20 @@ type OCSPConfig struct {
 	ResponderOverrides map[string]string
 }
 
+// CertificateEventData contains contextual information for
+// an obtained, renewed or revoked certificate.
+// EXPERIMENTAL: subject to change.
+type CertificateEventData struct {
+	// Domain or subject name of the certificate.
+	Name string
+
+	// Storage key for the issuer used for this certificate.
+	IssuerKey string
+
+	// Location in storage at which the certificate could be found.
+	StorageKey string
+}
+
 // certIssueLockOp is the name of the operation used
 // when naming a lock to make it mutually exclusive
 // with other certificate issuance operations for a