Addon manager: Reduce fetches from GitHub

Reduce the number of GitHub fetches when rebuilding the local addon cache by using a remote cache stored on FreeCAD's servers.

Intended to mitigate the Addon Manager hitting GitHub's rate limiters.

Addresses, but does not fully close, #15059

src/Mod/AddonManager/addonmanager_workers_startup.py

@@ -45,6 +45,7 @@ from AddonStats import AddonStats
 import NetworkManager
 from addonmanager_git import initialize_git, GitFailed
 from addonmanager_metadata import MetadataReader, get_branch_from_metadata
+import addonmanager_freecad_interface as fci
 
 translate = FreeCAD.Qt.translate