Use POCO instead of QNetworkAccessManager to download urls

[levkropp]

multipass find works, which means the new implementation can already successfully get data from https urls. I had to build with cmake ../ -DMULTIPASS_ENABLE_TESTS=off and packaging a snap without gui.

Immediate things to work on:

• SSL Certificate Verification is disabled for testing purposes (VERIFY_NONE)

  Poco::Net::Context::Ptr context = new Poco::Net::Context(
  Poco::Net::Context::CLIENT_USE,
  "",
  "",
  "",
  Poco::Net::Context::VERIFY_NONE,
  9,
  false,
  "ALL");

  https://docs.pocoproject.org/current/Poco.Net.Context.html

• Tests need to be remade for POCO

• multipass start and (in my experience) multipassd crashes instantly, no logs