[Identity] Add config for Identity service

Summary:
Depends on D3799

In order to implement PAKE registration/login in the Identity service, we need to persist a secret key and then share that key with the gRPC methods that use it. In this diff I have created a config module to load the key from a file into memory and then shared the key with the gRPC methods by adding a Config struct to MyIdentityService.

Test Plan: Ran the service and tested that the relevant Config was available in MyIdentityService

Reviewers: jimpo, karol-bisztyga, palys-swm, atul

Reviewed By: jimpo

Subscribers: ashoat, Adrian, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3812