[BUGFIX] #11484 QGIS server: embedded layers are not published (GetCapab...

...ilities, GetMap, etc)

Issue #1072 QGIS server: embedded groups do not render in GetMap requests if
 leave layers are requested has been fixed 40d91457f6f069886c5ba1c28db1efb2ff4ccac7

But another fix about embedded layers lacks. A fixme annotation is in the
 QgsServerProjectParser::createLayerFromElement. This one is about return the
 layer if it is embedded.

In QgsServerProjectParser::createLayerFromElement, we need to generate a
 QgsServerProjectParser of the parent project to return the mapLayer for
 embedded layers. To do it, I use QgsCacheConfig to get the
 QgsServerProjectParser for the parent project.

Funded by 3Liz.