Secure the http rest api

[pmqtt]

Describe the bug
Currently, everybody that knows the ip address and port of the broker can send api events
To Reproduce

$ curl -d '{"user":"test", "password":"1234"}' -H 'Content-Type: application/json' -X POST http://localhost:1884/rest/api/v0.1/create_user

Steps to solve
To solve the problem, extends the rest_api class in http_rest_server.cpp to validate an authentication token. Furthermore, generate at startup the token and write them to file.