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>

models/webhook/webhook.go

@@ -13,6 +13,8 @@ import (
 	"code.gitea.io/gitea/models/db"
 	"code.gitea.io/gitea/modules/json"
 	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/modules/secret"
+	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/timeutil"
 	"code.gitea.io/gitea/modules/util"
 
@@ -195,6 +197,9 @@ type Webhook struct {
 	Meta            string     `xorm:"TEXT"` // store hook-specific attributes
 	LastStatus      HookStatus // Last delivery status
 
+	// HeaderAuthorizationEncrypted should be accessed using HeaderAuthorization() and SetHeaderAuthorization()
+	HeaderAuthorizationEncrypted string `xorm:"TEXT"`
+
 	CreatedUnix timeutil.TimeStamp `xorm:"INDEX created"`
 	UpdatedUnix timeutil.TimeStamp `xorm:"INDEX updated"`
 }
@@ -401,6 +406,29 @@ func (w *Webhook) EventsArray() []string {
 	return events
 }
 
+// HeaderAuthorization returns the decrypted Authorization header.
+// Not on the reference (*w), to be accessible on WebhooksNew.
+func (w Webhook) HeaderAuthorization() (string, error) {
+	if w.HeaderAuthorizationEncrypted == "" {
+		return "", nil
+	}
+	return secret.DecryptSecret(setting.SecretKey, w.HeaderAuthorizationEncrypted)
+}
+
+// SetHeaderAuthorization encrypts and sets the Authorization header.
+func (w *Webhook) SetHeaderAuthorization(cleartext string) error {
+	if cleartext == "" {
+		w.HeaderAuthorizationEncrypted = ""
+		return nil
+	}
+	ciphertext, err := secret.EncryptSecret(setting.SecretKey, cleartext)
+	if err != nil {
+		return err
+	}
+	w.HeaderAuthorizationEncrypted = ciphertext
+	return nil
+}
+
 // CreateWebhook creates a new web hook.
 func CreateWebhook(ctx context.Context, w *Webhook) error {
 	w.Type = strings.TrimSpace(w.Type)