[C++][Pistache] Add some missing {{declspec}} on classes and functions

[sjoubert]

The {{declspec}} annotations were not consistently applied and were missing on some classes and free functions.

Side note: the declspec and defaultInclude config options are not officially documented for the cpp-pistache-server generator, but they work correctly (aside the missing annotations). Not sure if something needs to be done on that front.

My use case is that I'm building my server and APIs in a shared library and while trying to use the model classes in my tests couldn't link to it since some of the model symbols (to_json and from_json in my case) were not exported (I'm using declspec and defaultInclude for the classical use case of exporting symbols).

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