Add exporter for splunk hec reporting

[ChrisMandich]

Proposed changes

This change allows for Splunk HEC to be enabled as an Exporter for Nuclei with the following example config.

# splunkhec contains configuration options for splunkhec exporter
splunkhec:
  # Hostname for splunkhec instance
  host: <HEC domain>
  # Port is the port of splunkhec instance
  port: 8088
  # IndexName is the name of the splunkhec index
  index-name: nuclei
  # SSL enables ssl for splunkhec connection
  ssl: true
  # SSLVerification disables SSL verification for splunkhec
  ssl-verification: true
  # Username for the splunkhec instance
  token: <HEC Token>  

Docs - projectdiscovery/nuclei-docs#100

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)

[Mzack9999]

• code: lgtm
• files to restore: CODE_OF_CONDUCTS.md and SECURITY.MD

[ehsandeep]

@ChrisMandich thank you for adding splunk export support.

@Mzack9999 those files are now managed from https://github.com/projectdiscovery/.github