Integrate the new hash-sig interface for SecretKey

[shariqnaiyer]

Describe the feature

https://github.com/b-wagn/hash-sig/pull/89/files

The above PR on the hash-sig specs introduces a new interface for the SecretKey struct.

The functions introduced should be integrated into ream's post-quantum-crypto crate:

fn get_activation_interval(&self) -> Range<u64>
fn get_prepared_interval(&self) -> Range<u64>
fn get_prepared_interval(&self) -> Range<u64>

Additional context

No response