Confirm feature parity with secure_headers <=> rails vanilla

A discussion in https://github.com/twitter/secure_headers/issues/275 was about ensuring that rails gets support for feature policy and how it would require an API similar to the CSP API. It made me think we should evaluate current features and open issues to see what we'd also like to see in rails.

* [ ] Compile a list
* [ ] Review it
* [ ] open PRs/issues
* [ ] 🍨

I'd like to deprecate and/or transfer ownership of this library eventually :smile:

Comece pela discussão em secure_headers issue 275 e, em seguida, revise a lista atual de funcionalidades e as issues abertas do secure_headers e do suporte a Rails. Considera-se concluído quando a lista de paridade tiver sido compilada, revisada e os PRs ou issues resultantes tiverem sido abertos; o payload não especifica arquivos nem testes.

Escrita pelo modelo de indexação a partir do texto da issue.