Add create listening exercise to the AI service

[Kuroashi1995]

Create a method in the AI service with the following specs:

• [ ] createListeningExercise that takes two params: type and description
• [ ] create a prompt that takes the params and asks for a listening exercise in written form, as a JSON with the fields: Audio and the field Exercises
• [ ] #21
• [ ] parse the transcription and feed the parts to the generateAudio method
• [ ] concatenate the final audio
• [ ] save the audio