Fixes #18443: Introduce Webhook realm provider

[joshmue]

Generic realm provider notifying other services/realms about the creation or deletion of hosts via HTTP Webhooks. (See redmine ticket, too)

This provider is able to send custumizable HTTP POST-requests ("Webhook") to a HTTP-Server. E.g.

{"operation":"create","hostname":"host.lan","params":{"userclass":"User class",...}}

The HTTP-Server should respond with JSON, too. E.g. on creation:

{"randompassword": "foo"}

As Webhooks are not a strict standard, the HTTP headers can be extended and the JSON keys in the body are also customizable.

There is opt-in support for a request signing procedure described in https://developer.github.com/webhooks/securing/

[dmitri-d]

Thanks for your PR!

Recent changes in smart-proxy's realm module were implemented (in part) to allow to keep realm providers separate from the core code. I would suggest creating a dedicated repository for webhooks realm provider and packaging the code as a gem. This wouldn't require significant changes, mostly things to support gem packaging. You can find a bit more information at http://projects.theforeman.org/projects/foreman/wiki/How_to_Create_a_Smart-Proxy_Plugin#Bundler-Configuration and http://projects.theforeman.org/projects/foreman/wiki/How_to_Install_a_Smart-Proxy_Plugin.

Additionally, it might be useful to include in README docs links to projects, repositories, or example setups that use webhooks api to provide interface for realm services.

[joshmue]

Thank you for the reply,

I will start working on this 👍

Closing this PR in favor of the gem approach...