User registries should not require unique email IDs

Today API Connect has the following restrictions/requirements regarding user registries:

1. All users of the following kind must be registered with an email ID:

   1. users in local user registries

   2. consumer users (local or non-local user registries)

2. In any user registry (local or non-local), no 2 users may share the same email ID.

This was confirmed in PMR TS004756156.

The issue is that it is common to have multiple user ID in a registry that share the same email ID. The 2 most common reasons are the following:

1. In enterprises, it is common for system users to interact with API Connect, for example for automation purposes. It is not common to create unique email accounts for each system user. Instead it is more common to assign the email ID of the team that set up the system user, to the system user. Thus multiple system users might share the same email ID.

2. Companies are increasingly adopting the security principle of limiting privileged access by creating multiple user IDs for employees who have admin rights on systems--one user ID with privileged accesses and another one without. So a user would log into the Console Management interface with their unprivileged user when they just want to check status, etc., and they would log in with their privileged user when they want to make changes. Since both user IDs belong to the same employee and one would not create a separate email accounts for the privileged and unprivileged user IDs, both user IDs would be assigned the same email ID.

Also it is not uncommon that when an employee leaves a company, and a new employee with the same name is hired, then the email ID of the former employee will be assigned to the new employee. In such cases API Connect should be able to differentiate between the 2 user accounts. As this article shows, https://www.bbc.com/news/technology-24283179 at least one public email provider is re-using old email IDs for new users.