Add HTTP request header authentication to NBI

[markabrahams]

This PR provides an HTTP-header-based NBI authentication solution for the security issue raised in this forum post:
https://forum.genieacs.com/t/secure-rest-api-endpoints-of-genieacs-nbi/1093

The solution adds an optional configuration parameter "NBI_AUTHENTICATION_KEY", which - when present - is required to be present in an "x-api-key" HTTP header of any incoming request. When the "NBI_AUTHENTICATION_KEY" configuration parameter is absent, the current behaviour stands i.e. no authentication is required.

[markabrahams]

I'd also like to take the opportunity to thank all contributors to this great project - I was able to stand up GenieACS incredibly quickly and use it to debug some TR-069 client issues I was having. Nice work team!

[josineiaraujo]

Olá, eu tbm gostaria de ver algum tipo de autenticação na API....