adding support for dsl extractors

[Mzack9999]

Proposed changes

This PR adds support for DSL extractors

Checklist

• Pull request is created against the dev branch
• All checks passed (lint, unit/integration/regression tests etc.) with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

How to reproduce

Example template:

id: example

info:
  name: example
  author: example
  severity: info
  description: omfp
  tags: info

requests:
  - raw:
      - |
        GET / HTTP/1.1
        Host: {{Hostname}}

    extractors:
      - type: dsl
        dsl:
          - "len(body)"

Execution:

$ go run . -duc -no-interactsh -t target-length.yaml -u https://192.168.1.1

                     __     _
   ____  __  _______/ /__  (_)
  / __ \/ / / / ___/ / _ \/ /
 / / / / /_/ / /__/ /  __/ /
/_/ /_/\__,_/\___/_/\___/_/   2.6.8

                projectdiscovery.io

[WRN] Use with caution. You are responsible for your actions.
[WRN] Developers assume no liability and are not responsible for any misuse or damage.
[INF] Using Nuclei Engine 2.6.8 (outdated)
[INF] Using Nuclei Templates 8.9.4 (latest)
[INF] Templates added in last update: 20
[INF] Templates loaded for scan: 1
[2022-04-20 11:34:01] [example] [http] [info] https://192.168.1.1/ [7313]

Notes

• Depends on: Adding DSL extractor description + example nuclei-docs#34

[Ice3man543]

LGTM!

[forgedhallpass]

@Mzack9999 great addition, but please don't forget to update the documentation, and link the changes to this PR.