Implement `Auth` scan service request for authorizing access to a set of keys and their scan results

[arya2]

Motivation

We want a way to:

• Authenticate RPC clients/users
• Authorize access to viewing keys and their results

Possible Design

We could add basic username/password authentication for RPC clients before authorizing access to viewing keys and their results:

• Add an accounts db column where the keys are usernames, and the value is a hashed password + a list of registered keys for the account
• Accept either a fresh token or a username/password pair in the request
• Return a list of viewing keys that were registered by that account and a fresh token with the username as the response
• Prefix the keys in sapling results with usernames
• Add a config field for allowing unauthenticated requests

[arya2]

This is for later when we're working on authentication, it may be changed before we start on authentication.

[arya2]

@upbqdn @oxarbitrage Is this design okay, or do we want to do something else?

[upbqdn]

It looks good to me. I was thinking we could write a list of possible solutions in a discussion so that it's easier to tell we're picking the most suitable one. Will I start the discussion outlining the options?

[oxarbitrage]

It looks good to me but i am not sure if that is the simplest way. I will like to do more research before implementing, i don't think we have to decide now.