[draft][services][identity] Initial .proto file for identity service

Summary:
Problem statement: We need a way to authenticate users and let services verify a user's identity. Rather than have each service manage identities internally, we decided to create an Identity Service that users and other services talk to.

D2950 introduced authentication directly in the Backup Service. We had this notion of full auth and simple auth in that diff, where full auth meant signing in with a password or with an eth wallet, and simple auth meant using the backup ID (secret) to prove a user’s identity to the Backup Service. In this new design, the Identity Service replaces full auth with three APIs to register, login, and look up a user. The login API returns a random, unique token to the user. After 10 minutes of user inactivity, this token will expire. The user then presents this token to other services, which can query the Identity Service to validate it. Once validated, other services can use a service-specific shared secret to perform a “simple auth” going forward.

Functional requirements: This service needs to be able to register, login, and look up users.

Non-functional requirements: The Identity Service needs to be highly available, accessible only by authorized devices and services, and highly secure. If the Identity Service is down, users will not be able to create backups or recover their backup key.

Here is a sequence diagram illustrating a basic use case: https://www.figma.com/file/GF3xZAMmjpBMXrOIERmzQO/Identity-Service?node-id=13%3A14

Failure cases:

• User is not registered or not logged in, tries to create a new backup, Backup Service cannot find a valid token when it queries the Identity Service so it rejects the request

Corner cases:

Open questions:

• How should we ensure high availability?
• Which auth mechanism(s) should we use at the gRPC level?
• Does this design let us introduce new APIs like "RevokeToken" in the future?
• How should token be used? Should user just MAC a message with it when communicating with other services (e.g. backup)?

Depends on D3579

Test Plan: just a .proto file, so no testing required

Reviewers: ashoat, jimpo, karol-bisztyga, anunayk, palys-swm, geekbrother

Reviewed By: ashoat, karol-bisztyga, palys-swm

Subscribers: geekbrother, palys-swm, Adrian, atul, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3573