rename pay_parameters for no amount invoice

rename pay_parameters for no amount invoice

This commit renames the function `pay_parameters_for_zero_amount_invoice`
to `pay_parameters_for_variable_amount_invoice`.

The term "variable amount" is used to align with
the naming convention in the LDK node, helping to avoid
confusion between similar packages.

Fixes: https://github.com/lightningdevkit/rust-lightning/issues/2879
Replaces: https://github.com/lightningdevkit/rust-lightning/pull/2979
Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>