Add Webhook authorization header (#20926)

_This is a different approach to #20267, I took the liberty of adapting
some parts, see below_

## Context

In some cases, a weebhook endpoint requires some kind of authentication.
The usual way is by sending a static `Authorization` header, with a
given token. For instance:

- Matrix expects a `Bearer <token>` (already implemented, by storing the
header cleartext in the metadata - which is buggy on retry #19872)
- TeamCity #18667
- Gitea instances #20267
- SourceHut https://man.sr.ht/graphql.md#authentication-strategies (this
is my actual personal need :)

## Proposed solution

Add a dedicated encrypt column to the webhook table (instead of storing
it as meta as proposed in #20267), so that it gets available for all
present and future hook types (especially the custom ones #19307).

This would also solve the buggy matrix retry #19872.

As a first step, I would recommend focusing on the backend logic and
improve the frontend at a later stage. For now the UI is a simple
`Authorization` field (which could be later customized with `Bearer` and
`Basic` switches):


![2022-08-23-142911](https://user-images.githubusercontent.com/3864879/186162483-5b721504-eef5-4932-812e-eb96a68494cc.png)

The header name is hard-coded, since I couldn't fine any usecase
justifying otherwise.

## Questions

- What do you think of this approach? @justusbunsi @Gusted @silverwind 
- ~~How are the migrations generated? Do I have to manually create a new
file, or is there a command for that?~~
- ~~I started adding it to the API: should I complete it or should I
drop it? (I don't know how much the API is actually used)~~

## Done as well:

- add a migration for the existing matrix webhooks and remove the
`Authorization` logic there


_Closes #19872_

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Gusted <williamzijl7@hotmail.com>
Co-authored-by: delvh <dev.lh@web.de>

services/webhook/deliver_test.go

@@ -5,10 +5,16 @@
 package webhook
 
 import (
+	"context"
 	"net/http"
+	"net/http/httptest"
 	"net/url"
 	"testing"
+	"time"
 
+	"code.gitea.io/gitea/models/db"
+	"code.gitea.io/gitea/models/unittest"
+	webhook_model "code.gitea.io/gitea/models/webhook"
 	"code.gitea.io/gitea/modules/setting"
 
 	"github.com/stretchr/testify/assert"
@@ -38,3 +44,38 @@ func TestWebhookProxy(t *testing.T) {
 		}
 	}
 }
+
+func TestWebhookDeliverAuthorizationHeader(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+
+	done := make(chan struct{}, 1)
+	s := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		assert.Equal(t, "/webhook", r.URL.Path)
+		assert.Equal(t, "Bearer s3cr3t-t0ken", r.Header.Get("Authorization"))
+		w.WriteHeader(200)
+		done <- struct{}{}
+	}))
+	t.Cleanup(s.Close)
+
+	hook := &webhook_model.Webhook{
+		RepoID:      3,
+		URL:         s.URL + "/webhook",
+		ContentType: webhook_model.ContentTypeJSON,
+		IsActive:    true,
+		Type:        webhook_model.GITEA,
+	}
+	err := hook.SetHeaderAuthorization("Bearer s3cr3t-t0ken")
+	assert.NoError(t, err)
+	assert.NoError(t, webhook_model.CreateWebhook(db.DefaultContext, hook))
+
+	hookTask := &webhook_model.HookTask{HookID: hook.ID, EventType: webhook_model.HookEventPush}
+
+	assert.NoError(t, Deliver(context.Background(), hookTask))
+	select {
+	case <-done:
+	case <-time.After(5 * time.Second):
+		t.Fatal("waited to long for request to happen")
+	}
+
+	assert.True(t, hookTask.IsSucceed)
+}