The schemagen tool and graphqlservice library should be aware of the version at runtime

[wravery]

Currently there's a version.txt file which is only used to determine the project version in CMake, and it's kept in synch with the latest release tag. There are several places where it would make sense to integrate that into the binaries as well:

• The schemagen tool should implement a --version flag which reports the release it was built from.
• Generated files should include the version they were built against and report an error/warning if used with an incompatible graphqlservice library at compile time, indicating that they should be regenerated. At runtime they might throw an exception.
• On Windows, all of the binaries (exe and dll) should have a version number stamp in the file metadata.