[server] add public key column for sessions table

Summary: We need to associate and persist a public key for each session for libolm.

Test Plan: Ran migration script locally and verified that the public_key column was created in the sessions table.

Reviewers: atul, karol-bisztyga, ashoat

Reviewed By: ashoat

Subscribers: benschac, ashoat, palys-swm, Adrian

Differential Revision: https://phabricator.ashoat.com/D2934