Implement pure python AES-IGE mode

[SWORDIntel]

AES-IGE Pure Python Implementation

• AES Generic Base Class: The previous AES256 class inside src/crypto_standalone/symmetric/aes.py has been updated and renamed to a base AES class. The _key_expand and initialization steps were updated to natively support 16, 24, and 32 byte keys for AES-128, AES-192, and AES-256 respectively, adapting the number of key expansion iterations based on the key length.
• Backward Compatibility: A subclass AES256 that inherits from AES was placed inside src/crypto_standalone/symmetric/aes.py that specifically throws a ValueError for key lengths other than 32 bytes to ensure existing legacy implementations are completely unaffected.
• AES-IGE Strategy: Developed a dedicated mode class AESIGE which conforms to the conventions of other block cipher modes (like AEAD implementations). Added aes_ige_encrypt and aes_ige_decrypt functions to provide user-friendly encapsulation, enforcing the necessary length requirements, utilizing a two-block combined IV ($C_0 || P_0$), handling standard recursion behavior, and rejecting any padded/invalid length requirements.
• Framework integration: The new classes and helper functions were successfully attached to module exports inside src/crypto_standalone/symmetric/__init__.py and the main __init__.py export. Updated the README.md to advertise the AES-IGE features.
• Validation: Included comprehensive Kat (known-answers-test) test suites utilizing multi-block output tests with vectors derived from independent implementations inside a new tests/unit/test_aes_ige.py. Tests verify correct functionality and error handling. Code passes validation (linting and testing).

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PR created automatically by Jules for task 7188788929984145490 started by @SWORDIntel

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