v2: Consider moving SSL-related options to a wrapped SSLContext helper

[shazow]

(Placeholder braindump from our team meeting on 2020-12-16)

We have a bunch of SSL-related options that are confusing (server_hostname, assert_hostname, etc).

If we wanted to make a bigger change for v2 to change the signatures in a meaningful way, we can consider adding an SSLContext-like object in urllib3 which wraps the stdlib SSLContext, but also move all of our backports into that code and add all of the SSL-adjacent functions.