feat: add project instructions as a valid parameter

[henriquepb-lilt]

This pull request introduces a new instructions field to the LanguagePair model in lilt/models/language_pair.py and updates related test files to incorporate this change. The most important changes include adding the new field, updating the model's constructor and property methods, and modifying test cases to validate the new functionality.

Model Updates:

• Added a new instructions field to the LanguagePair model, including updates to the swagger_types and attribute_map dictionaries, and the __init__ method to accept this field. (lilt/models/language_pair.py) [1] [2] [3] [4]
• Implemented getter and setter methods for the instructions property to handle the new field. (lilt/models/language_pair.py)

Test Updates:

• Updated test/test_job_create_parameters.py to include the instructions field in the language_pairs parameter for both optional and non-optional test cases. [1] [2] [3]
• Updated test/test_language_pair.py to include the instructions field in the LanguagePair test instances. [1] [2]