Replace the abbreviated term "padd" with either "padding" or "pad" throughout
the encryption module. While "padd" was originally chosen to align with other
variable names ("type" and "mode"), using standard terminology improves code
readability and resolves codespell warnings.
Additionally, refactor relevant code to use C++ structured bindings for cleaner
implementation.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closes scylladb/scylladb#23251