Adds support for configuring HTTP Feature Policy (#33439)

A HTTP feature policy is Yet Another HTTP header for instructing the
browser about which features the application intends to make use of and
to lock down access to others. This is a new security mechanism that
ensures that should an application become compromised or a third party
attempts an unexpected action, the browser will override it and maintain
the intended UX.

WICG specification: https://wicg.github.io/feature-policy/

The end result is a HTTP header that looks like the following:

```
Feature-Policy: geolocation 'none'; autoplay https://example.com
```

This will prevent the browser from using geolocation and only allow
autoplay on `https://example.com`. Full feature list can be found over
in the WICG repository[1].

As of today Chrome and Safari have public support[2] for this
functionality with Firefox working on support[3] and Edge still pending
acceptance of the suggestion[4].

#### Examples

Using an initializer

```rb
# config/initializers/feature_policy.rb
Rails.application.config.feature_policy do |f|
  f.geolocation :none
  f.camera      :none
  f.payment     "https://secure.example.com"
  f.fullscreen  :self
end
```

In a controller

```rb
class SampleController < ApplicationController
  def index
    feature_policy do |f|
      f.geolocation "https://example.com"
    end
  end
end
```

Some of you might realise that the HTTP feature policy looks pretty
close to that of a Content Security Policy; and you're right. So much so
that I used the Content Security Policy DSL from #31162 as the starting
point for this change.

This change *doesn't* introduce support for defining a feature policy on
an iframe and this has been intentionally done to split the HTTP header
and the HTML element (`iframe`) support. If this is successful, I'll
look to add that on it's own.

Full documentation on HTTP feature policies can be found at
https://wicg.github.io/feature-policy/. Google have also published[5] a
great in-depth write up of this functionality.

[1]: https://github.com/WICG/feature-policy/blob/master/features.md
[2]: https://www.chromestatus.com/feature/5694225681219584
[3]: https://bugzilla.mozilla.org/show_bug.cgi?id=1390801
[4]: https://wpdev.uservoice.com/forums/257854-microsoft-edge-developer/suggestions/33507907-support-feature-policy
[5]: https://developers.google.com/web/updates/2018/06/feature-policy

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