Custom header support for webhooks

[esep]

Issue description

In the SCM Manager Webhook page, I can currently input the URL with HTTP method, but no headers such as api keys that would be required by e.g. Fisheye and other APIs. Because of this, I have to use other hook methods, such as curl commands.

For example, see:
https://confluence.atlassian.com/fisheye/configuring-commit-hooks-960155647.html

Feature Request

Problem to be solved

I would like to be able to add custom headers to the requests sent by the webhook.

Proposed solution

Support for adding/removing custom headers (key-value pairs) should be added to the webhook page (js React files) so that they would be passed on to the request that is sent in the class:
https://github.com/scm-manager/scm-webhook-plugin/blob/develop/src/main/java/sonia/scm/webhook/impl/AhcWebHookHttpClient.java