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/utils/hook.go

@@ -72,18 +72,39 @@ func CheckCreateHookOption(ctx *context.APIContext, form *api.CreateHookOption)
 func AddOrgHook(ctx *context.APIContext, form *api.CreateHookOption) {
 	org := ctx.Org.Organization
 	hook, ok := addHook(ctx, form, org.ID, 0)
-	if ok {
-		ctx.JSON(http.StatusCreated, convert.ToHook(org.AsUser().HomeLink(), hook))
+	if !ok {
+		return
 	}
+	apiHook, ok := toAPIHook(ctx, org.AsUser().HomeLink(), hook)
+	if !ok {
+		return
+	}
+	ctx.JSON(http.StatusCreated, apiHook)
 }
 
 // AddRepoHook add a hook to a repo. Writes to `ctx` accordingly
 func AddRepoHook(ctx *context.APIContext, form *api.CreateHookOption) {
 	repo := ctx.Repo
 	hook, ok := addHook(ctx, form, 0, repo.Repository.ID)
-	if ok {
-		ctx.JSON(http.StatusCreated, convert.ToHook(repo.RepoLink, hook))
+	if !ok {
+		return
 	}
+	apiHook, ok := toAPIHook(ctx, repo.RepoLink, hook)
+	if !ok {
+		return
+	}
+	ctx.JSON(http.StatusCreated, apiHook)
+}
+
+// toAPIHook converts the hook to its API representation.
+// If there is an error, write to `ctx` accordingly. Return (hook, ok)
+func toAPIHook(ctx *context.APIContext, repoLink string, hook *webhook.Webhook) (*api.Hook, bool) {
+	apiHook, err := convert.ToHook(repoLink, hook)
+	if err != nil {
+		ctx.Error(http.StatusInternalServerError, "ToHook", err)
+		return nil, false
+	}
+	return apiHook, true
 }
 
 func issuesHook(events []string, event string) bool {
@@ -135,6 +156,11 @@ func addHook(ctx *context.APIContext, form *api.CreateHookOption, orgID, repoID
 		IsActive: form.Active,
 		Type:     form.Type,
 	}
+	err := w.SetHeaderAuthorization(form.AuthorizationHeader)
+	if err != nil {
+		ctx.Error(http.StatusInternalServerError, "SetHeaderAuthorization", err)
+		return nil, false
+	}
 	if w.Type == webhook.SLACK {
 		channel, ok := form.Config["channel"]
 		if !ok {
@@ -185,7 +211,11 @@ func EditOrgHook(ctx *context.APIContext, form *api.EditHookOption, hookID int64
 	if err != nil {
 		return
 	}
-	ctx.JSON(http.StatusOK, convert.ToHook(org.AsUser().HomeLink(), updated))
+	apiHook, ok := toAPIHook(ctx, org.AsUser().HomeLink(), updated)
+	if !ok {
+		return
+	}
+	ctx.JSON(http.StatusOK, apiHook)
 }
 
 // EditRepoHook edit webhook `w` according to `form`. Writes to `ctx` accordingly
@@ -202,7 +232,11 @@ func EditRepoHook(ctx *context.APIContext, form *api.EditHookOption, hookID int6
 	if err != nil {
 		return
 	}
-	ctx.JSON(http.StatusOK, convert.ToHook(repo.RepoLink, updated))
+	apiHook, ok := toAPIHook(ctx, repo.RepoLink, updated)
+	if !ok {
+		return
+	}
+	ctx.JSON(http.StatusOK, apiHook)
 }
 
 // editHook edit the webhook `w` according to `form`. If an error occurs, write
@@ -254,6 +288,12 @@ func editHook(ctx *context.APIContext, form *api.EditHookOption, w *webhook.Webh
 	w.Release = util.IsStringInSlice(string(webhook.HookEventRelease), form.Events, true)
 	w.BranchFilter = form.BranchFilter
 
+	err := w.SetHeaderAuthorization(form.AuthorizationHeader)
+	if err != nil {
+		ctx.Error(http.StatusInternalServerError, "SetHeaderAuthorization", err)
+		return false
+	}
+
 	// Issues
 	w.Issues = issuesHook(form.Events, "issues_only")
 	w.IssueAssign = issuesHook(form.Events, string(webhook.HookEventIssueAssign))