ci: add basic macOS CI

Part-of: <gstreamer/orc!46>
5 jobs for macos-ci in 11 minutes and 1 second (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #3714276
debian sid

00:01:25

passed #3714280
gst-macos-10.15
meson macos

00:00:33

passed #3714279
gstreamer-windows 1809 windows docker
msys2

00:07:38

passed #3714277
gstreamer-windows 1809 windows docker
vs2017 amd64

00:00:59

passed #3714278
gstreamer-windows 1809 windows docker
vs2017 x86

00:00:58