demo: added xcconfig for code sign

Configs/App-Shared.xcconfig

@@ -2,6 +2,7 @@
 // https://help.apple.com/xcode/#/dev745c5c974
 
 #include "Module-Shared.xcconfig"
+#include "Codesign.xcconfig"
 
 // Name of an asset catalog app icon set whose contents will be merged into the `Info.plist`.
 ASSETCATALOG_COMPILER_APPICON_NAME = AppIcon

Configs/Codesign.xcconfig

@@ -0,0 +1 @@
+// You can set code sign related xcconfig in these file for debugging/testing, like CODE_SIGN_IDENTITY

Configs/Test-Shared.xcconfig

@@ -2,6 +2,7 @@
 // https://help.apple.com/xcode/#/dev745c5c974
 
 #include "Module-Shared.xcconfig"
+#include "Codesign.xcconfig"
 
 // This is a list of paths to folders to be searched by the compiler for included or imported header files when compiling C, Objective-C, C++, or Objective-C++. Paths are delimited by whitespace, so any paths with spaces in them need to be properly quoted.
 HEADER_SEARCH_PATHS = $(inherited) "$(SRCROOT)/../SDWebImage/Private"