[native] use short form for prekeys

Summary: parse the prekey blob string using folly and then convert the std::string to a rust::String / jsi::String

Test Plan: https://gist.github.com/vdhanan/1b93f4b98a04c1ff0c2c3c7074b4c625

Reviewers: kamil, marcin

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11312