Add entropy detection to random number generation

[VolodymyrBg]

This commit implements entropy detection for the random number generation process in the ecall_generate_random function. The changes include:

1. Added a has_sufficient_entropy function that calculates Shannon entropy and performs frequency analysis on the generated random bytes
2. Implemented additional heuristic checks for sequential bytes and repeating patterns
3. Added detection of suspicious byte distributions to ensure high-quality randomness
4. Generated random values are now verified for entropy quality before being used
5. Removed the TODO comment as the feature is now implemented

These improvements enhance the security of the random number generation process by ensuring high-quality entropy and rejecting potentially weak random values.