chore(gpu): refactor cuda radix ciphertext

[andrei-stoian-zama]

Big refactoring of the CudaRadixCiphertextFFI struct:

• step 1: add the new CudaRadixCiphertext class, and make the backend internal classes use it.

[github-actions]

✅ Backward-compat snapshot: everything looks good! No backward-compatibility issues detected.

[github-actions]

Forward Compatibility Matrix

TYPE	1.5.5 loads 1.6.3 data	1.5.5 loads 1.7.0 data	1.6.3 loads 1.7.0 data	1.5.5 loads nightly data	1.6.3 loads nightly data	1.7.0 loads nightly data
CompactPublicKey	FAIL: invalid value: integer 1, expected variant index 0 <= i < 1	FAIL: invalid value: integer 1, expected variant index 0 <= i < 1	OK	FAIL: invalid value: integer 1, expected variant index 0 <= i < 1	OK	OK
CompactPkeCrs	OK	OK	OK	OK	OK	OK
ProvenCompactCiphertextList	FAIL: invalid value: integer 1, expected variant index 0 <= i < 1	FAIL: invalid value: integer 1, expected variant index 0 <= i < 1	OK	FAIL: invalid value: integer 1, expected variant index 0 <= i < 1	OK	OK