Extending http variables list with dns generated variables

[Mzack9999]

Proposed changes

This PR extends the list of variables for HTTP protocol with DNS ones generated from the domain name (ref #1174)

• {{FQDN}} - Fully qualified domain name (already supported)
• {{RDN}} - Root domain name
• {{DN}} - Domain name
• {{TLD}} - Toplevel domain
• {{SD}} - Sub domain

and here is how it can be interpreted using an example as www.projectdiscovery.io

Global Variable	Example value
{{FQDN}}	www.projectdiscovery.io
{{RDN}}	projectdiscovery.io
{{DN}}	projectdiscovery
{{TLD}}	io
{{SD}}	www

Note:

• {{SD}} - subdomains include lower-level subdomains as well, for example in the case of test.abc.domain.com, {{SD}} will be evaluated as test.abc

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)

[Ice3man543]

lgtm!