[frost] allow share index in share backup to vary to any size

[LLFourn]

https://github.com/LLFourn/secp256kfun/blob/86e2908bc4f123463c14fa4559c742fce2ba55c5/schnorr_fun/src/share_backup.rs

Only allows the share backup to be 1 u5 long or 52 u5s. I think if we're going to make it variable length then it should be able to be any length.