[keepercore] Add http command to support web hooks

[aquamatthias]

Why

While it is possible to attach to the event bus, or to query the state of the system, it might be the simplest solution to also allow for web hooks to be triggered.
In order to support this scenario, a http command should be implemented, which will perform the request on execution.
This way it would be possible to trigger web hooks based on events (or any other trigger) in the system.

What

• Add an http command that allows to specify: method, url
• Every element in the in_stream will trigger a http request
• When the method is POST: send the element as body
• Allow the definition of parallel requests per command

Open Questions

• How to handle auth?

Acceptance Criteria

TBD