Allow to show response headers in json output without using -irh option

[JoshuaMart]

Hi,
This PR add the -irh / -include-response-headers option to display the response headers without having to use -irr

This option allow to reduces the output size as -irr also includes the response body, which can considerably increase the size of the JSON if many targets are scanned.

I'm also taking advantage of this PR to pluralize some variables concerning headers.

[dogancanbakir]

LGTM

$ go run . -u scanme.sh -json -irh | jq .

    __    __  __       _  __
   / /_  / /_/ /_____ | |/ /
  / __ \/ __/ __/ __ \|   /
 / / / / /_/ /_/ /_/ /   |
/_/ /_/\__/\__/ .___/_/|_|
             /_/

                projectdiscovery.io

[INF] Current httpx version v1.3.4 (latest)
{
  "timestamp": "2023-09-01T11:29:38.211469631Z",
  "hash": {
    "body_md5": "444bcb3a3fcf8389296c49467f27e1d6",
    "body_mmh3": "-2088429648",
    "body_sha256": "2689367b205c16ce32ed4200942b8b8b1e262dfc70d9bc9fbc77c49699a4f1df",
    "body_simhash": "590632390773907579",
    "header_md5": "61f86158e2e9d13ba5b291bfff646c08",
    "header_mmh3": "-703836329",
    "header_sha256": "2011c02f1d5542dda7089129117589fc639608cd3a9812e85c9b5818ecef36fc",
    "header_simhash": "9814386056708734847"
  },
  "port": "443",
  "url": "https://scanme.sh",
  "input": "scanme.sh",
  "scheme": "https",
  "content_type": "text/plain",
  "method": "GET",
  "host": "128.199.158.128",
  "path": "/",
  "headers": {
    "content_length": "2",
    "content_type": "text/plain; charset=utf-8",
    "date": "Fri, 01 Sep 2023 11:29:38 GMT"
  },
  "time": "1.153600667s",
  "a": [
    "128.199.158.128",
    "2400:6180:0:d0::91:1001"
  ],
  "words": 1,
  "lines": 1,
  "status_code": 200,
  "content_length": 2,
  "failed": false,
  "knowledgebase": {
    "PageType": "nonerror"
  }
}