[Windows] [mmap_data_loader.cpp] mmap equivalent for Windows (#8916)

* [Windows Build] Implement MMAP for mmap_data_loader.cpp

There is no sys/mman.h or posix-compatible mmap() implementation
on Windows.  The extension data loaders use it to map in data
files, so adding an implementation.

Test-run,

   & .\cmake-out\extension\data_loader\test\Debug\extension_data_loader_test.exe --gtest_brief=1 --gtest_filter=MmapDataLoader*
   Running main() from ...\executorch\third-party\googletest\googletest\src\gtest_main.cc
   [==========] 8 tests from 1 test suite ran. (50 ms total)
   [  PASSED  ] 8 tests.

* apply code suggestions

* fix src -> srcs typo

* try to fix build

* fix src/headers brackets

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Co-authored-by: Jeff Whiteside <jwhites@microsoft.com>