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:

先從 secure_headers issue 275 中的討論開始，然後檢視 secure_headers 和 Rails 支援目前的功能清單及開放的 issue。完成表示編製對等性清單、檢視該清單，並開啟由此產生的 PRs 或 issues；payload 未指定任何特定檔案或測試。

由索引模型根據 Issue 內容生成。