Add bearer/basic auth option to webhooks

Notifications > Webhooks section needs optional bearer/basic auth

What you need to do is to add two optional fields below Webhook URL:

- Authentication Type (dropdown):
- None (default)
- Basic Auth
- Bearer Token

If Basic Auth is selected, ask for:
- Username
- Password

If Bearer Token is selected, ask for:
- Token input field

That is it. Additionally when sending the webhook request, do the following:

- For basic Auth:
- Encode username:password in base64.
- Add header: Authorization: Basic base64(username:password)

- For bearer token, it's more simple.
- Add header: `Authorization: Bearer YOUR_TOKEN`

Start by locating the Notifications > Webhooks settings UI and the code that sends webhook requests. Trace how webhook configuration is stored and passed to the sender, then implement the three authentication modes and verify that the resulting request headers match the issue requirements.

Written by the indexing model from the issue text.