feat(fs, config): add support for custom filesystem type construction (#9887)

For Synctrain I would like to create a virtual filesystem that exposes
iOS' photo library. This can only be accessed through APIs.

lib/versioner/external.go

@@ -74,7 +74,7 @@ func (v external) Archive(filePath string) error {
 	}
 
 	context := map[string]string{
-		"%FOLDER_FILESYSTEM%": v.filesystem.Type().String(),
+		"%FOLDER_FILESYSTEM%": string(v.filesystem.Type()),
 		"%FOLDER_PATH%":       v.filesystem.URI(),
 		"%FILE_PATH%":         filePath,
 	}

lib/versioner/staggered_test.go

@@ -139,10 +139,12 @@ func TestCreateVersionPath(t *testing.T) {
 	}
 
 	folderCfg := config.FolderConfiguration{
-		ID:   "default",
-		Path: tmpDir,
+		ID:             "default",
+		FilesystemType: config.FilesystemTypeBasic,
+		Path:           tmpDir,
 		Versioning: config.VersioningConfiguration{
 			Type:   "staggered",
+			FSType: config.FilesystemTypeBasic,
 			FSPath: versionsDir,
 		},
 	}