Query trusted web servers for time

[fhunleth]

It's possible to set the time by using the Date header in HTTPS requests. While this won't be near as accurate as NTP, it should be plenty good for TLS certificate expiration checks.

See the following links for examples of this method:

1. sdwdate
2. secure-time-date

The second link is basically a curl command and a regex to extract the date field:

$ curl -sI --tlsv1.2 -s https://nerves-project.org
HTTP/2 200
server: GitHub.com
content-type: text/html; charset=utf-8
last-modified: Tue, 04 Aug 2020 10:59:08 GMT
access-control-allow-origin: *
etag: "5f293f7c-ea"
expires: Thu, 07 Jan 2021 08:24:26 GMT
cache-control: max-age=600
accept-ranges: bytes
date: Thu, 07 Jan 2021 13:13:57 GMT
age: 0
content-length: 234

This feature would be intended as an alternative to using NTP, so #77 would be nice to have first.