Support for SDK auth token association with email

[cloutiertyler]

John: Unclear what this means.

Do we want to be able to do effectively spacetime identity set-email from the client, to enable token recovery? If so, why - I wouldn't generally expect a user to have access to one long hex string but have lost the other.

Do we want to extend the local-storage (save_credentials/load_credentials) interfaces to store a mapping from email to creds, rather than creds alone? If so, why - client authors can do whatever they want with identity/token pairs; what lengths should we go to in offering convenient interfaces for common patterns? Is associating tokens with emails a common pattern for clients?

Tyler: It means each SDK should have a setEmail function.