feat(core): add variable Scalar type to PBS for input and output

[IceTDrinker]

closes: https://github.com/zama-ai/tfhe-rs-internal/issues/526

PR content/description

As indicated in the previous PR easier to do for the PBS as scalar types don't really interact

Check-list:

• [x] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been added / updated (for bug fixes / features)
• [x] Relevant issues are marked as resolved/closed, related issues are linked in the description
• [x] Check for breaking changes (including serialization changes) and add them to commit message following the conventional commit specification