Add MacOS XPlatCpp Target

[nassosterz-ms]

Changes

• Developers can now build a static lib for MacOS with the introduced XPlatCpp_MacOS build target
• Renamed Build/Ios to Build/Apple to better reflect purpose of this directory
• Renamed also existing projects to reflect that the xcode project is for both iOS and MacOS
• Small changes in the source code mostly to guard code for macOS
• Added schemes so that static libs are generated as Release (previously they were Debug flavor).
• Code compiles and libXplatCpp_MacOS.a static library is generated. (in XPlatCppSDK repo, in this repo, for master branch iOS is not building).

Sanity check that iOS also is generated properly and that test app builds - need a valid json to run it - if I can get one I can test it (relates to XPlatCppSdk)

[amccalib]

@AlonsoMondal Can you run this through the XplatCpp build pipeline and make sure things look okay?

[AlonsoMondal]

Hey, sorry about the late response, we had an issue with running generation using a custom branch for the template repo.
Run is looking good, here is a PR for us to look at the result and see if everything looks good:
PlayFab/XPlatCppSdk#51

PS: PR above should be abandoned once we're done here.