Add support for CSP level 3 about secure_headers

[oreoshake]

Hi @ankitagrawal0x90 I think there's some support for CSP3 goodies. Have you noted which contributions would be necessary? I'm happy to review/release more support.

[ankitagrawal0x90]

@oreoshake Thank you for your response, appreciate it. I will look through the code to find CSP 3 directives currently supported by the gem and share more.

[ankitagrawal0x90]

I read through the code https://github.com/github/secure_headers/blob/main/lib/secure_headers/headers/content_security_policy_config.rb and https://github.com/github/secure_headers/blob/main/lib/secure_headers/headers/policy_management.rb and the library has everything I need.

I was partly mislead by the documentation here https://www.rubydoc.info/gems/secure_headers/6.3.2 here which mentions

Content Security Policy (CSP) - Helps detect/prevent XSS, mixed-content, and other classes of attack. CSP 2 Specification

Thanks for all the great work. Closing this issue.