httpsrc: Port to new subclassing API

Fixes gstreamer/gst-plugins-rs#50
3 jobs for master in 5 minutes and 37 seconds (queued for 1 second)
Status Job ID Name Coverage
  Lint
passed #70756
rustfmt

00:00:30

 
  Test
passed #70758
allowed to fail
test nightly

00:05:06

passed #70757
test stable

00:04:49