Add support for additional headers with minemeld.ft.http.HttpFT module

[spenceation]

Is your feature request related to a problem?

Certain applications require an API-KEY or authentication through the HTTP headers. This is currently supported with the minemeld.ft.json.SimpleJSON module.

Describe the solution you'd like

The HTTP module should allow for additional headers to be provided. This will allow support for more applications.

Describe alternatives you've considered

An alternative is to implement a gateway in between that handles the authentication, but this is not an ideal solution.

Additional context

Gitlab specifically requires API tokens to be passed through the HTTP header. This will allow for more internal GitOps functionality of Minemeld.