Add post_generate to Ciphersuite trait

[conradoplg]

The new Ciphersuite trait methods introduced with the Taproot ciphersuite could allow making the RedPallas ciphersuite (from reddsa crate) easier to use by ensuring even-Y pubkeys using e.g. post_dkg(). (Currently the user needs to do it themselves if they use it with frost-core, rather than the non-generic functions in reddsa). However, unfortunately, it's not possible to run code after trusted dealer key generation.

Add a new Ciphersuite method, e.g. post_generate or post_split (since it should be effectively be called at the end of split()), which will then allow the RedPallas simplification.