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>

modules/structs/hook.go

@@ -18,12 +18,13 @@ var ErrInvalidReceiveHook = errors.New("Invalid JSON payload received over webho
 
 // Hook a hook is a web hook when one repository changed
 type Hook struct {
-	ID     int64             `json:"id"`
-	Type   string            `json:"type"`
-	URL    string            `json:"-"`
-	Config map[string]string `json:"config"`
-	Events []string          `json:"events"`
-	Active bool              `json:"active"`
+	ID                  int64             `json:"id"`
+	Type                string            `json:"type"`
+	URL                 string            `json:"-"`
+	Config              map[string]string `json:"config"`
+	Events              []string          `json:"events"`
+	AuthorizationHeader string            `json:"authorization_header"`
+	Active              bool              `json:"active"`
 	// swagger:strfmt date-time
 	Updated time.Time `json:"updated_at"`
 	// swagger:strfmt date-time
@@ -43,19 +44,21 @@ type CreateHookOption struct {
 	// enum: dingtalk,discord,gitea,gogs,msteams,slack,telegram,feishu,wechatwork,packagist
 	Type string `json:"type" binding:"Required"`
 	// required: true
-	Config       CreateHookOptionConfig `json:"config" binding:"Required"`
-	Events       []string               `json:"events"`
-	BranchFilter string                 `json:"branch_filter" binding:"GlobPattern"`
+	Config              CreateHookOptionConfig `json:"config" binding:"Required"`
+	Events              []string               `json:"events"`
+	BranchFilter        string                 `json:"branch_filter" binding:"GlobPattern"`
+	AuthorizationHeader string                 `json:"authorization_header"`
 	// default: false
 	Active bool `json:"active"`
 }
 
 // EditHookOption options when modify one hook
 type EditHookOption struct {
-	Config       map[string]string `json:"config"`
-	Events       []string          `json:"events"`
-	BranchFilter string            `json:"branch_filter" binding:"GlobPattern"`
-	Active       *bool             `json:"active"`
+	Config              map[string]string `json:"config"`
+	Events              []string          `json:"events"`
+	BranchFilter        string            `json:"branch_filter" binding:"GlobPattern"`
+	AuthorizationHeader string            `json:"authorization_header"`
+	Active              *bool             `json:"active"`
 }
 
 // Payloader payload is some part of one hook