Currently leap seconds are hard-coded into the C, as SOFA does. However, there are use cases (e.g. astropy/astropy#5783) where it would be useful to have the option for leap seconds to be added at runtime. This option should be added.

The main debate here is whether or not this is acceptable, as it's a break from how SOFA does it. I think yes, because it's a pretty important use case - users should be able to get leap seconds from online sources and update at runtime without having to recompile...