* Add CodeQL GitHub code scanning workflow
* Only compile main sources for code scanning
* Move test .proto files to test sources
`annotations.proto` also seems to be only relevant for tests because the test
explicitly registers them as extensions. By default the Proto adapter does not
consider them.
* Address some code scanning findings
* Fix some more findings
* Avoid NPE in PostConstructAdapterFactory
The RawType's Superclass might be null. This happens, for example, when the type is a collection.
* Add test case for NPE in PostConstructAdapterFactory
* Improve the code quality of PostConstructAdapterFactoryTest
* Improve the code quality of PostConstructAdapterFactoryTest
* Improve the code quality of PostConstructAdapterFactoryTest
