Allow user accounts to be merged if the username/email already exists

Original issue in the Drupal queue filed and fixed by codebymikey. I've quoted the issue below and I'll submit a PR based on that patch with a few tweaks needed for the Backdrop implementation.

Problem/Motivation

simpleSAMLphp auth should be capable of merging with existing Drupal user accounts without crashing when a username/email conflict occurs.

It throws the following:

<?php PDOException: SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry 'my-username' for key 'name': INSERT INTO {users} (uid, name, pass, created, access, status, init, uuid) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4, :db_insert_placeholder_5, :db_insert_placeholder_6, :db_insert_placeholder_7); Array ( [:db_insert_placeholder_0] => 16602 [:db_insert_placeholder_1] => redacted [:db_insert_placeholder_2] => redacted [:db_insert_placeholder_3] => 1610106922 [:db_insert_placeholder_4] => 1610106922 [:db_insert_placeholder_5] => 1 [:db_insert_placeholder_6] => redacted [:db_insert_placeholder_7] => redacted ) in drupal_write_record() (line 7460 of /app/application/web/includes/common.inc). ?>

Steps to reproduce

Example use case:

User A has an existing Drupal account with username usera and email code>usera@example.com</code.
simpleSAMLphp should be able to detect that user during the login process rather than crashing because the email or username is already in use.

Proposed resolution

Provide configuration options for administrators to decide what to do during the situation. Either merge/abort the login.

And if there's an edge case where two accounts exist (account 1 with the username, and account 2 with the email, then it should abort for the admin to clean up).

Remaining tasks

Provide a patch/PR.

backdrop-contrib / simplesamlphp_auth