Playground for cpr extensions

See https://github.com/libcpr/cpr

Tries the following:

• Factory to store named sets of standard Session configurations (baseURL, Header, Parameters, Redirects, HTTP proxies)
• All Session objects created share DNS, connection, SSL and cookie cache
• Sessions are configured with a retry policy with backof
• Can invoke verbs (Get, etc) with relative URLs, otherwise same parameters as cpr
• Proxy autodiscovery via libproxy
• proxy connectivity test on session creation
• during request retries optionally try connects w/o proxy

It provides a class cprex::Session utilizing cpr::Session.

No MultiPerform yet.

Basic use:

cprex::Factory::PrepareSession("ipify", "https://api64.ipify.org");
cprex::Factory::PrepareSession("stat", "https://httpstat.us/");
...
auto ipify = cprex::Factory::CreateSession("ipify");
auto r = ipify.Get("/");
...
auto stat = cprex::Factory::CreateSession("stat");
r = stat.Get("/200");

Some parameter:

r = stat.Get("/200", cpr::Parameters {{"sleep", "5000"}});

TODOs:

• maybe resolve IP in PrepareSession() and also maybe perform connectivity tests
• Add decorrelation jitter as described here:
  • https://github.com/App-vNext/Polly/wiki/Retry-with-jitter
  • https://www.pollydocs.org/strategies/retry
  • https://github.com/Polly-Contrib/Polly.Contrib.WaitAndRetry/tree/master
  • https://github.com/Polly-Contrib/Polly.Contrib.WaitAndRetry/blob/master/src/Polly.Contrib.WaitAndRetry/Backoff.DecorrelatedJitterV2.cs