Created ordered list for HTTP headers

[nakulbajaj]

Modified ZGrab2's HTTP package to extract headers as a list of
HeaderField objects and then extract as a map via the textproto package.

Created new HeaderField object with UTF-8 string for header names
paired with a byte slice for header values.

[mzpqnxow]

@nakulbajaj does this change how the headers are represented in the NDJSON output?

(Aside from ordering, of course)

[nakulbajaj]

@mzpqnxow This doesn't change the "headers" dictionary as part of the JSON output at all. Instead, it creates a new field in the JSON output called "header_list," which composed of a list of header objects (which include a name and a value string).