Add config option for cookies digest

You can now configure custom digest for cookies in the same way as `serializer`:

  config.action_dispatch.cookies_digest = 'SHA256'
Merge pull request #16467 from strzalek/cookies-digest-config-option2

Cookies digest config option (pt. 2)

Conflicts:
	actionpack/CHANGELOG.md
	actionpack/lib/action_dispatch/middleware/cookies.rb