add options to specify User-Agent in headless template

[parrasajad]

Proposed changes

user_agent: random | off | default | custom

example template:

id: extract-urls

info:
  name: Extract URLs from HTML attributes
  author: dwisiswant0
  severity: info
  tags: headless,extractor

headless:
  - steps:
      - args:
          url: "{{BaseURL}}"
        action: navigate
      - action: waitload
      - action: script
        name: extract
        args:
          code: |
            '\n' + [...new Set(Array.from(document.querySelectorAll('[src], [href], [url], [action]')).map(i => i.src || i.href || i.url || i.action))].join('\r\n') + '\n'

    user_agent: custom
    custom_user_agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/74.0.3729.169 Safari/537.36

    extractors:
      - type: kval
        part: extract
        kval:
          - extract

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)

[sonarcloud]

SonarCloud Quality Gate failed.   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
1 Code Smell

0.0% Coverage
0.0% Duplication