Update auth package to allow persisted queries

[emmatown]

This is to allow a Keystone setup where persisted queries are required for all callers except the Admin UI.

To make that approach work, the mutation that the Admin UI uses for authentication naturally needs to use a persisted query since it's not authenticated yet. This makes two small changes to @keystone-6/auth to make it possible to use it:

• createAuth returns a persistedQueries function that returns a map of a sha256 hash to the exact query the front-end uses so it can be used in where you define
• Uses a minimal fetch call for the sign in mutation that adds the appropriate extensions.persistedQuery while still passing the actual query in so this doesn't
  • I went with fetch directly rather than getting Apollo Client to send extensions.persistedQuery since I don't think we want to this for any other query and it seemed like unnecessary complexity

Note this doesn't use persisted queries for all Admin UI queries, I don't think this would be very practical or helpful since those queries are very dynamic so it would be a massive number of queries that essentially allows anything.

In a PR stacked on top of this, I've added an example that actually uses persisted queries in the way described above.