write callback and termination API

[edwinpjacques]

Description

1. Allow write callback. Application was to use it to leverage the Kubernetes watch API which maintains a long-running connection.
2. API to terminate connection from another thread.
3. Fix all the disabled/broken CI tests that block the merge.

Checklist

Not all of these might apply to your change but the more you are able to check
the easier it will be to get your contribution merged.

• CI passes
• Description of proposed change
• Documentation (README, code doc blocks, etc) is updated
• Existing issue is referenced if there is one
  • write callback support #162 write callback support
  • CI does not work locally #161 CI does not work locally
• Unit tests for the proposed change

[edwinpjacques]

@mrtazz I am ready for your review, and hopefully your approval! I did a lot of work to get the CI working well again. I enabled all tests, even the OPTIONS test. I would very much appreciate if this can be approved, or if you can provide any necessary feedback for approval ASAP. Thank you.

[edwinpjacques]

I thought that my changing of the squid container was perhaps superfluous, as the old container works locally. However, it does not work in CI. I switched back to the container I provided and my custom configuration file, as that passed locally and in CI.

[edwinpjacques]

@shawkinsl would you be able to provide any feedback on this merge request? It fixes CI and allows for stream processing of a GET response via write callback.

[edwinpjacques]

squashed commits, ready for approval.

[edwinpjacques]

@ElonDusk26 and @KaiPetzke would you mind to give a "thumbs up" on this review so I can get it merged with a clean conscience? Thanks!

[ElonDusk26]

Astounding work @edwinpjacques! Hopefully @mrtazz agrees with me.

[edwinpjacques]

Got out-of-band permission to merge from @mrtazz based on trust. Thanks for the kudos @ElonDusk26 ! I hope you can help fix the issue #158. It would be nice to have that resolved in the next release of code. Thank you.

[KaiPetzke]

Sorry to disagree on this one. The callback declaration:

auto writeCallback = [](void *data, size_t size, size_t nmemb, void *userdata) -> size_t

looks too much old-style C-ish to me. This is a C++ wrapper around libcurl, and we should use C++ APIs, not C APIs.

The C++11 style would be to use std::function.

The Pre-C++11-style would be to use a class with a virtual function callback