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.go

@@ -90,7 +90,12 @@ func Deliver(ctx context.Context, t *webhook_model.HookTask) error {
 	case http.MethodPut:
 		switch w.Type {
 		case webhook_model.MATRIX:
-			req, err = getMatrixHookRequest(w, t)
+			txnID, err := getMatrixTxnID([]byte(t.PayloadContent))
+			if err != nil {
+				return err
+			}
+			url := fmt.Sprintf("%s/%s", w.URL, url.PathEscape(txnID))
+			req, err = http.NewRequest("PUT", url, strings.NewReader(t.PayloadContent))
 			if err != nil {
 				return err
 			}
@@ -130,6 +135,16 @@ func Deliver(ctx context.Context, t *webhook_model.HookTask) error {
 	req.Header["X-GitHub-Event"] = []string{event}
 	req.Header["X-GitHub-Event-Type"] = []string{eventType}
 
+	// Add Authorization Header
+	authorization, err := w.HeaderAuthorization()
+	if err != nil {
+		log.Error("Webhook could not get Authorization header [%d]: %v", w.ID, err)
+		return err
+	}
+	if authorization != "" {
+		req.Header["Authorization"] = []string{authorization}
+	}
+
 	// Record delivery information.
 	t.RequestInfo = &webhook_model.HookRequest{
 		URL:        req.URL.String(),