(core) Add authSubject and authProvider to sessions

Summary:
This also updates Authorizer to link the authSubject
to Grist users if not previously linked. Linked subjects
are now used as the username for password-based logins,
instead of emails, which remain as a fallback.

Test Plan: Existing tests, and tested login flows manually.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3356

app/server/lib/requestUtils.ts

@@ -21,6 +21,7 @@ export const TEST_HTTPS_OFFSET = process.env.GRIST_TEST_HTTPS_OFFSET ?
 const INTERNAL_FIELDS = new Set([
   'apiKey', 'billingAccountId', 'firstLoginAt', 'filteredOut', 'ownerId', 'gracePeriodStart', 'stripeCustomerId',
   'stripeSubscriptionId', 'stripePlanId', 'stripeProductId', 'userId', 'isFirstTimeUser', 'allowGoogleLogin',
+  'authSubject',
 ]);
 
 /**