* 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
Now the thrown exception carries this information:
java.lang.IllegalArgumentException: Invalid attempt to bind an instance of java.lang.Integer as a @JsonAdapter for com.google.gson.functional.JsonAdapterAnnotationOnClassesTest$D. @JsonAdapter value must be a TypeAdapter, TypeAdapterFactory, JsonSerializer or JsonDeserializer.
Johannes Frohnmeyer
Marcono1234