Add support for custom headers in webhook API

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What does this MR do and why?

Add support for custom headers in webhook API

Relates to Support for custom webhook headers in the API (#455528 - closed)

MR acceptance checklist

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Screenshots or screen recordings

API only changes

How to set up and validate locally

1. Create a project hook using the API

   {
     "url": "some-url",
     "custom_headers": [{ "key": "some-key", "value": "some-value" }]
   }

2. Open the UI and check that a hook with a some-key header has been set, test the webhook and see if the value is passed correctly
3. Edit the hook using the API

   { "custom_headers": [{ "key": "some-other-key", "value": "some-value" }] }

   Now, the hook should have both some-key and some-other-key custom headers.
4. Use the set custom header endpoint: PUT /projects/:id/hooks/:hook_id/custom_headers/some-key

   { "value": "some-new-value" }

   Test the hook again and verify that the new value is sent in the header
5. Delete a custom header: DELETE /projects/:id/hooks/:hook_id/custom_headers/some-key. You can check in the UI that the custom header was removed.
6. Repeat these steps for groups (if you have an active license)