[Identity] Script to generate and persist a server keypair for PAKE

Summary:
Depends on D3747

The Identity service will need to persist a keypair to use for PAKE registration and login. We use the ciphersuite implementation from our common library to generate a random keypair, then write the secret key (public key can be derived from this) to secrets/secret_key.

Test Plan:
Ran the keygen executable a couple times to make sure the directory is created if it doesn't exist and the file contents are overwritten each time

varun@varuns-MBP identity % ./target/debug/keygen
Creating secrets directory "/Users/varun/Code/comm/services/identity/secrets"
Writing secret key to "/Users/varun/Code/comm/services/identity/secrets/secret_key"
varun@varuns-MBP identity % ./target/debug/keygen
Writing secret key to "/Users/varun/Code/comm/services/identity/secrets/secret_key"

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

Reviewed By: jimpo, ashoat

Subscribers: ashoat, Adrian, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3799