gstreamer1.0-plugins-good: use libsoup-3.0 by default

- add a PACKAGECONFIG for soup3
- add PACKAGECONFIG_SOUP variable as it is used in webkitgtk and epiphany

Signed-off-by: Markus Volk <f_l_k@t-online.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>