Add signing/encoder client functionality

[peterschwarz]

The client features of C++ SDK should be updated to match the functionality of the Rust SDK. Specifically this means adding a module for creating/encoding Batches and Transactions, as well as a signing module which can sign, verify, generate private keys, and generate public keys.

Add any appropriate unit tests for the new functionality.

(Duplicated by https://jira.hyperledger.org/browse/STL-462)