Need a migration guide for moving from 3.x to 4.x

[wravery]

The migration from 3.x to 4.x is challenging. I've been meaning to start a migration guide based on a couple of samples I've migrated. Here's a list of topics I think should be covered, essentially an outline for the doc (will update with more as they come up):

• C++17 to C++20:
  • Describe the toolchain requirements with links to the code and samples.
  • Describe how to use custom Awaitables with links to the unit tests.
  • Describe how to implement field accessors as a coroutine.
• Removing inheritance and replacing with type-erased wrappers:
  • Change the class declaration, potentially add std::enable_shared_from_this<>.
  • Make override or final field accessors non-virtual.
  • Wrapping type-erased type objects with interface and union objects.
  • Decoupling an inheritance hierarchy from service::Object (Chance of 3.X.X style generator being added as option to 4.X.X? #210)
• Simplify the field accessor signatures:
  • Return wrapped types directly (unless implementing a coroutine).
  • Make methods const or non-const as appropriate (can also get rid of a lot of mutable member variables this way).
  • Pass parameters by const& reference, && r-value reference, or value.
  • Drop unused service::FieldParams arguments.
• Using the new CMake functions:
  • ⚠️ Potentially destructive, build the schema in a sub-directory and use source control (i.e. Git) before and after (CMake: update_graphql_..._files functions can delete files without warning if misused #212).
  • Migrating from the unified output to what would have been generated with --separate-files
  • Removing the graphqlintrospection target (graphqlservice target no longer works without graphqlintrospection #196)
• Links to migration commits in sample and external projects:
  • gqlmapi
  • electron-cppgraphql
  • samples in this repo (e.g. replacing all of the "unified" schemas with --separate-files versions, converting TodayMock.* implementations, etc.)