Suggestion: support slh-dsa

[gkgoat1]

I am developing an authorization system to augment an end-to-end encrypted user system. The existing encryption system uses slh-dsa for signatures because it was proven correct provided the security of the hash function used, so aupporting it in biscuits would help the augmentation.
One note is that slh-dsa signatures are far larger than elliptic curve signatures, so biscuits embedding them would be substantially larger. Otherwise, it seems like a drop-in replacement for the uses of elliptic curve signatures here, and could be another supported signature format.