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>

routers/api/v1/repo/hook.go

@@ -69,7 +69,11 @@ func ListHooks(ctx *context.APIContext) {
 
 	apiHooks := make([]*api.Hook, len(hooks))
 	for i := range hooks {
-		apiHooks[i] = convert.ToHook(ctx.Repo.RepoLink, hooks[i])
+		apiHooks[i], err = convert.ToHook(ctx.Repo.RepoLink, hooks[i])
+		if err != nil {
+			ctx.InternalServerError(err)
+			return
+		}
 	}
 
 	ctx.SetTotalCountHeader(count)
@@ -112,7 +116,12 @@ func GetHook(ctx *context.APIContext) {
 	if err != nil {
 		return
 	}
-	ctx.JSON(http.StatusOK, convert.ToHook(repo.RepoLink, hook))
+	apiHook, err := convert.ToHook(repo.RepoLink, hook)
+	if err != nil {
+		ctx.InternalServerError(err)
+		return
+	}
+	ctx.JSON(http.StatusOK, apiHook)
 }
 
 // TestHook tests a hook