* 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
- Lets you customize the proto field name serialization per field
- Lets you use different cases for serialization (e.g. you could use UpperCamelCase for your proto field names and willing to serialize them using LowerHyphen)
- Lets you choose to serialize enum values with their value (0, 1, 2, ...) instead of their name.
