libcamera says that cameras should default to manual focus mode. This
means that unless pipewire clients specifically change this control,
users with an autofocus-capable camera are left with an out-of-focus
image. This patch sets the autofocus mode to continuous and enables
auto-exposure (as the default for this is unspecified).
Testing with an imx708 on Raspberry Pi OS on a Raspberry Pi 4, before
this patch the image was generally out of focus in Firefox/webrtc, after
this patch autofocus works correctly.