refactor(concrete_core): change internal api of lwe ciphertext

[aPere3]

Resolves:

closes zama-ai/concrete-core-internal#291

Description

This PR is an example of how the api change proposed by @mayeul-zama could be brought to the library incrementally. Here we apply the new structure on the LweCiphertext entity.

Checklist

(Use '[x]' to check the checkboxes, or submit the PR and then click the checkboxes)

• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)
• [ ] The PR description links to the related issue (to link an issue, use '#XXX'.)
• [ ] The tests on AWS have been launched and are successful (comment with @slab-ci cpu_test and/or @slab-ci gpu_test to trigger the tests)
• [ ] The draft release description has been updated
• [ ] Check for breaking changes (including serialization changes) and add them to commit message following the conventional commit specification