Operation request: RC6 Encrypt / RC6 Decrypt

## Summary
RC6 encryption, just like the existing AES encryption
### Example Input
flag{68f25cc8-1a9f-40e8-ac3b-a85982a52f8f}
Key(Hex): 46 53 5a 33 36 66 33 76 55 38 73 35 04 04 04 04
Mode: CBC
IV: WcE4Bbm4kHYQsAcX
Input: Raw
Output: base64
### Example Output
RKCTaz+fty1J2qsz4DI6t9bmMiLBxqFrpI70fU4IMemczIlM+z1IoVQobIt1MbXF

Start by locating the existing AES encryption operation referenced in the issue and compare its inputs, modes, key handling, and output behavior. Implement the corresponding RC6 encrypt/decrypt operation with CBC mode and verify that the supplied key, IV, raw input, and base64 output reproduce the example.

Written by the indexing model from the issue text.