Allow setting sourceProject on groups

[u-cav]

Description

Related to #601.

It is now possible to correctly set sourceRepository, thanks!
SourceProject is automatically shown for embedded layers, but does not seem to be saved in the cfg. It is not possible to manually specify the sourceProject:

This makes total sense for layers, but for groups where groupAsLayer is true setting the sourceProject enables Lizmap to use the shared cache.

Setting this manually in the cfg works correctly, and the shared cache is used.

Enabling the field SourceProject for groups would enable users to use a centralized cache for the tiled baselayers where multiple layers are grouped as one.

CC: @mind84

Context

• Lizmap Web Client : 3.8.3
• Lizmap plugin : 4.4.4
• QGIS Desktop : 3.34.3
• QGIS Server : 3.34.12
• Py-QGIS-Server : 1.9.2
• QGIS Server plugin atlasprint : 3.4.1
• QGIS Server plugin lizmap_server : 2.11.2
• QGIS Server plugin wfsOutputExtension : 1.8.2