Adding a UserDataDir flag for headless executions

[marwan38]

I have a use case where I would like to execute nuclei headless templates using a custom user data directory for the browser.
I have the feature already added in my fork, but, I didn't want to put up a PR without opening up a discussion to see if the maintainers would want it included and/or if there is a better way of implementing it.

Nuclei allows for us to pass in flags for the browser executable (headless-flags or something, I believe). Meaning we could pass in --user-data-dir, but that would not work as nuclei executes in incognito by default.

My (perhaps naive solution) was adding a new flag --user-data-dir to nuclei cli itself, which disables incognito and launches the browser with the flag set. The alternative (and perhaps less invasive) solution would be listening to that flag and disabling incognito when it's there.