Improve cryptographic utilities and testing for cross-platform compatibility

[RomanTsisyk]

• Added generateKey() method to AESEncryption for secure AES key generation.
• Replaced android.util.Base64 with java.util.Base64 in RSAEncryption for JVM compatibility.
• Fixed byte array comparisons in AES tests using assertArrayEquals.
• Added Bouncy Castle as a provider for SHA256withRSA/PSS in DigitalSignature.
• Improved DigitalSignature to handle unsupported algorithms gracefully.
• Registered Bouncy Castle provider to support advanced cryptographic algorithms.
• Enhanced test coverage for AES, RSA, and DigitalSignature modules.
• Fixed test dependencies and ensured platform-independent testing.