[CommCoreModule] Add sessionSignature & newSession boilerplate to NetworkModule

Summary: NA, boilerplate (should've been squashed with previous Client diffs)

Test Plan: Continues to build, able to call

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

Reviewed By: ashoat

Subscribers: Adrian, benschac

Differential Revision: https://phabricator.ashoat.com/D3152