[CommCoreModule] Fill out sessionSignature(deviceID) => toSign boilerplate in Client

Summary: So we can hit SessionSignature(...) endpoint

Test Plan: Hit tunnelbroker with request and see if we get a response

Reviewers: ashoat, def-au1t, palys-swm, varun, karol-bisztyga, jimpo

Reviewed By: ashoat, varun

Subscribers: Adrian, benschac

Differential Revision: https://phabricator.ashoat.com/D3143