Update generate_infinite_expiry_signing_key.py

[volosianko77]

1. Created a generate_signing_key function to encapsulate the logic for generating the signing key and related data.
2. Moved the main logic into a main function.
3. Used input to get the account private key from the user instead of hardcoding it.
4. Improved function and variable names for better readability.
5. Added docstrings to functions to explain their purpose.
6. Modified the script to be more modular and easily extensible.