[Windows] Adds a native windows filesystem implementation

[davinci26]

Signed-off-by: Sotiris Nanopoulos sonanopo@microsoft.com

Commit Message:

Refactor filesystem_impl to use win32 APIs instead of POSIX subsystem. Fixes issue #11655.

Additional Description:

For easier review the major changes are the following:

1. Adds a Win32 impl
2. Strips away the file_shared_impl because it relied on errno for error handling. This change cascades to the POSIX implementation
3. Adds a preliminary error mapping so we don't rely on error strings.

Risk Level: Low
Testing: Already tested in filesystem_impl_test
Docs Changes: N/A
Release Notes: N/A

[davinci26]

cc: @envoyproxy/windows-dev

gimme one CI run to clean up any formatting or cascading build errors

[wrowe]

A couple quick comments for discussion, but generally looking really good here, thanks!

[davinci26]

Attempting to bump coverage above threshold. Technically it shouldn't have been affected by the change since the code is a null-op in linux.

[davinci26]

clang-tidy fails due to #13162 that is coming from upstream

[wrowe]

LGTM!

[davinci26]

@envoyproxy/senior-maintainers can I also get a review from you.

[mattklein123]

Nice cleanup. Thanks!