Add multiprocessing and log output to generator

[treiher]

This adds a simple form of multiprocessing to the code generation. The multiprocessing reduces the code generation for the current SPDM specification from 3:40 min to 1:20 min. There is still much room for improvement, but this would require more effort.

Log output is added to be able to observe the progress of the code generation.

The generator code is also refactored to ease maintaining the code:

• Unify how ada entities are imported (i.e., always import them directly), to make it easier to move functions between different generator modules
• Move message-related code generator functions to generator.message