Shake function or(and) map_to_field for transcript

[Valeh2012]

Hash2Field algorithm requires either extended message digest or extensible output function (XOF) according to https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-hash-to-curve-11#section-5.4

h2f.hpp already implements expand_message_xmd but not expand_msg_xof, primarily due to no XOF implementation in crypto3 suite. SHAKE is such XOF based on Keccak permutations.