Version 4.0 Release Candidate

This contains all of the planned code changes for 4.0 discussed in #172. I tested with local changes to the https://github.com/microsoft/vcpkg port file and https:://github.com/microsoft/gqlmapi. I still need to update the documentation before I merge into main and make an official release. Until then, comparing the next branch of gqlmapi to main can give you a preview of some of the changes.

What's Changed

• Add default fallthrough cases by @wravery in #166
• Add a learn_star_wars sample based on https://graphql.org/learn/ by @wravery in #168
• Merge work in progress from personal fork to start working on 4.0 by @wravery in #173
• More progress on 4.0 tasks by @wravery in #174
• More progress on 4.0 tasks by @wravery in #175
• Include fewer/smaller headers to speed up build by @wravery in #176
• Remove the schemagen --merge-files option and make everything use separate files by @wravery in #177
• Update to October 2021 release of the GraphQL spec by @wravery in #179
• Implement delivery by SubscriptionKey and a type-erased JSON writer interface by @wravery in #180
• Allow field getters to return a std::shared_ptr by @wravery in #181
• Distinguish AwaitableScalar from AwaitableObject by @wravery in #183
• Only install new CMake files for install configs by @wravery in #184
• Minor fixes from testing with the vcpkg port and gqlmapi by @wravery in #185

Full Changelog: v3.6.0...v4.0.0-rc