WIP - Add http header resource

[SimonBaeumer]

Add http header support, i.e.:

http:
    http://google.com:
        status: 200
        headers:
            key: 
            - value1
            - value2
        request-headers:
            key:
            - value1
            - value2

DoD:

• [x] Add headers to config
• [x] Validate header
• [x] Send headers in request
• [x] Write integration tests
• [x] Write unit tests
• [x] Check / beautify outputs
• [x] Add documentation
• [x] CLI add feature