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>

tests/integration/api_repo_hook_test.go

@@ -0,0 +1,47 @@
+// Copyright 2022 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package integration
+
+import (
+	"fmt"
+	"net/http"
+	"testing"
+
+	repo_model "code.gitea.io/gitea/models/repo"
+	"code.gitea.io/gitea/models/unittest"
+	user_model "code.gitea.io/gitea/models/user"
+	api "code.gitea.io/gitea/modules/structs"
+	"code.gitea.io/gitea/tests"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestAPICreateHook(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+
+	repo := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{ID: 37})
+	owner := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: repo.OwnerID})
+
+	// user1 is an admin user
+	session := loginUser(t, "user1")
+	token := getTokenForLoggedInUser(t, session)
+	completeURL := func(lastSegment string) string {
+		return fmt.Sprintf("/api/v1/repos/%s/%s/%s?token=%s", owner.Name, repo.Name, lastSegment, token)
+	}
+	req := NewRequestWithJSON(t, "POST", completeURL("hooks"), api.CreateHookOption{
+		Type: "gitea",
+		Config: api.CreateHookOptionConfig{
+			"content_type": "json",
+			"url":          "http://example.com/",
+		},
+		AuthorizationHeader: "Bearer s3cr3t",
+	})
+	resp := MakeRequest(t, req, http.StatusCreated)
+
+	var apiHook *api.Hook
+	DecodeJSON(t, resp, &apiHook)
+	assert.Equal(t, "http://example.com/", apiHook.Config["url"])
+	assert.Equal(t, "Bearer s3cr3t", apiHook.AuthorizationHeader)
+}