Expose certificate information on HTTPS responses

[alex]

For scraper I'm working on, I'd like to have access to the certificate supplied by the server for HTTPS requests. As far as I can tell, this is not currently exposed on either requests or responses.

[redapple]

Hey @alex , sorry for the late feedback.
There's a related issue about logging TLS options, cipher suites (#2111)

I haven't really looked into how we could expose that, but it'd probably be somewhere in the TLS options object that we pass to Twisted via the context factory.