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:

Start with the discussion in secure_headers issue 275, then review the current feature list and open issues for secure_headers and Rails support. Done means compiling the parity list, reviewing it, and opening the resulting PRs or issues; the payload names no specific files or tests.

索引モデルが issue の本文から書いたものです。