webhooks: add "user created" webhook

Webhooks send a POST to an admin-supplied URL when something happens,
with relevant information sent in JSON. One has been added for creating
users in Settings > Webhooks > User Created.

Lazily, the portion of GetUsers which generates a respUser has been
factored out, and is called to send the JSON payload.

A stripped-down common.Req method has been added, which is used by the
barebones WebhookSender struct.

webhooks.go

@@ -0,0 +1,38 @@
+package main
+
+import (
+	"net/http"
+	"net/url"
+	"time"
+
+	"github.com/hrfee/jfa-go/common"
+	"github.com/hrfee/jfa-go/logger"
+	lm "github.com/hrfee/jfa-go/logmessages"
+)
+
+type WebhookSender struct {
+	httpClient     *http.Client
+	timeoutHandler common.TimeoutHandler
+	log            *logger.Logger
+}
+
+// SetTransport sets the http.Transport to use for requests. Can be used to set a proxy.
+func (ws *WebhookSender) SetTransport(t *http.Transport) {
+	ws.httpClient.Transport = t
+}
+
+func NewWebhookSender(timeoutHandler common.TimeoutHandler, log *logger.Logger) *WebhookSender {
+	return &WebhookSender{
+		httpClient: &http.Client{
+			Timeout: 10 * time.Second,
+		},
+		timeoutHandler: timeoutHandler,
+		log:            log,
+	}
+}
+
+func (ws *WebhookSender) Send(uri string, payload any) (int, error) {
+	_, status, err := common.Req(ws.httpClient, ws.timeoutHandler, http.MethodPost, uri, payload, url.Values{}, nil, true)
+	ws.log.Printf(lm.WebhookRequest, uri, status, err)
+	return status, err
+}