Let API create and edit system webhooks, attempt 2 (#33180)

This PR fixes inconsistencies between system and default webhooks in the
Gitea API. (See also #26418)
- A system webhook is a webhook that captures events for all
repositories.
- A default webhook is copied to a new repository when it is created. 

Before this PR `POST /api/v1/admin/hooks/` creates default webhooks (if
not configured otherwise) and `GET /api/v1/admin/hooks/` returns system
webhooks.

The PR introduces an optional query parameter to `GET
/api/v1/admin/hooks/` to enable selecting if either default, system or
both kind of webhooks should be retrieved. By default the flag is set to
return system webhooks keep current behaviour.

## Examples

### System Webhooks

#### Create

```
POST /api/v1/admin/hooks/

{
  "type": "gitea",
  "active": false,
  "branch_filter": "*",
  "events": [ "create", "..." ],
  "config": {
    "url": "http://...",
    "content_type": "json",
    "secret": "secret",
    "is_system_webhook": true // <-- controls hook type
  }
}
```

#### List
```
GET/api/v1/admin/hooks?type=system //type argument is optional here since it's the default
```

#### Others
The other relevant endpoints work as expected by referencing the hook by
id
```
GET /api/v1/admin/hooks/:id
PATCH /api/v1/admin/hooks/:id
DELETE /api/v1/admin/hooks/:id
```


### Default Webhooks

#### Create
```
POST /api/v1/admin/hooks/

{
  "type": "gitea",
  "active": false,
  "branch_filter": "*",
  "events": [ "create", "..." ],
  "config": {
    "url": "http://...",
    "content_type": "json",
    "secret": "secret",
    "is_system_webhook": false // optional, as false is the default value
  }
}
```

#### List
```
GET/api/v1/admin/hooks?type=default
```

#### Others
The other relevant endpoints work as expected by referencing the hook by
id
```
GET /api/v1/admin/hooks/:id
PATCH /api/v1/admin/hooks/:id
DELETE /api/v1/admin/hooks/:id
```

models/webhook/webhook_system_test.go

@@ -0,0 +1,37 @@
+// Copyright 2017 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package webhook
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/models/db"
+	"code.gitea.io/gitea/models/unittest"
+	"code.gitea.io/gitea/modules/optional"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestGetSystemOrDefaultWebhooks(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+
+	hooks, err := GetSystemOrDefaultWebhooks(db.DefaultContext, optional.None[bool]())
+	assert.NoError(t, err)
+	if assert.Len(t, hooks, 2) {
+		assert.Equal(t, int64(5), hooks[0].ID)
+		assert.Equal(t, int64(6), hooks[1].ID)
+	}
+
+	hooks, err = GetSystemOrDefaultWebhooks(db.DefaultContext, optional.Some(true))
+	assert.NoError(t, err)
+	if assert.Len(t, hooks, 1) {
+		assert.Equal(t, int64(5), hooks[0].ID)
+	}
+
+	hooks, err = GetSystemOrDefaultWebhooks(db.DefaultContext, optional.Some(false))
+	assert.NoError(t, err)
+	if assert.Len(t, hooks, 1) {
+		assert.Equal(t, int64(6), hooks[0].ID)
+	}
+}