PANDO-71: Show errors for marked classes that don't meet the criteria for having a serializer generated

[schultzcole]

• Adds the source generator (SerializerIncrementalGenerator) which currently does the following:
  • finds types that have the marker attribute
  • validates them to ensure they meet the criteria
  • if they do not, emit a diagnostic
  • currently it also generates a basic utility class that outputs valid types, however this is not currently used anywhere (and will be removed once we start adding the actual source generation).
• Also adds unit test project that tests various error cases to ensure that they produce the correct diagnostic.
• Adds a stub project for generator integration tests. doesn't do anything currently, but is set up correctly to use the source generator