Feature/improved headers and cookies

[GThurler]

• Added cookie arguments to all functions, so we can use the --cookie flag in curl
• Added an override to the hard-coded application/json content type header, since some servers expect proprietary extra parameters to this value, e.g.: application/json;enp
• Implemented a CurtHttpHeaders class to store response headers in the same way the SDK's io package does, storing multiple values for each header - this was not an option in the http package's Request class since it uses a Map<String, String>.
• Implemented a CurtResponse class to replace the current usage of the Response class from http package - it behaves the same way we were using it, with added cookies 🍪