FindPython: On Windows, Enhance python debug version support

Add the following variables:
* Python_EXECUTABLE_DEBUG
* Python_INTERPRETER
* Python_DEBUG_POSTFIX

and target Python::InterpreterDebug.

python_add_library() command Manage DEBUG_POSTFIX target property based
on the value of Python_DEBUG_POSTFIX variable.

Fixes: #25874

Tests/FindPython/Python2/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.5)
+cmake_minimum_required(VERSION 3.15)
 
 project(TestPython2 LANGUAGES C)
 
@@ -39,7 +39,7 @@ target_compile_definitions (spam2 PRIVATE PYTHON2)
 
 add_test (NAME python2_spam2
           COMMAND "${CMAKE_COMMAND}" -E env "PYTHONPATH=$<TARGET_FILE_DIR:spam2>"
-          "${Python2_EXECUTABLE}" -c "import spam2; spam2.system(\"cd\")")
+          "${Python2_INTERPRETER}" -c "import spam2; spam2.system(\"cd\")")
 
 add_test(NAME findpython2_script
          COMMAND "${CMAKE_COMMAND}" -DPYTHON_PACKAGE_NAME=Python2