API changes for code reusage in utbot-python and other languages

[tamarinvs19]

• There are a lot of methods in the project that could be common for different languages but they depend on UtMethod and UtTestCase (contains UtMethod and JimpleBody) which are designed only for Kotlin/Java. We would like them to be something like interfaces with different implementations for different languages.

  Because of this we cannot reuse almost anything from code generator.

  • There are also several fields for mocking in CgContext that must be initialized but for now we don't use mocking in Python. We would like them to be nullable (at least for now).
  • There is a field classUnderTest: ClassId that shouldn't be mandatory for Python (JS, Go).

• Another problem in API that we have found is the class Instruction from CoverageApi.kt. It needs className but in Python a lot of functions are defined outside of classes. Probably there are other classes with the same issue.