Add a dummy TLS implementation to be used as fallback

[palant]

I’ve added a dummy TLS implementation to be used when neither OpenSSL nor BoringSSL are selected, providing only the APIs actually used by Pingora. This is a step towards solving two issues:

1. Pingora currently cannot compile without TLS support, yet sometimes an HTTP-only server is sufficient.
2. Someone aiming to provide another TLS implementation (e.g. [ feature request ] Use rustls #29) has no way of knowing which parts of the vast OpenSSL API actually have to be reproduced.

This at least gets Pingora to compile with --no-default-features. It will still fail at startup because it attempts to initialize some TLS connectors, some more work is required here. Also, quite a few tests rely on TLS being available – these probably need to run conditionally.

[johnhurt]

@palant, I should have marked this as Accepted weeks ago. It should be coming out in the weekly sync today or next week at the latest. You know the drill, once it does, I'll mark this as closed. Thanks for all your excellent contributions!

[johnhurt]

Hey, this has been merged ... Or it will be in a few minutes. Thanks!

[eaufavor]

Synced, thanks!