Allow deleting secondary email addresses from the account

## What feature would you like to see?

If a user has multiple email addresses linked to his/her account, allow deleting any of the secondary email address.

## Describe the feature you would like to add, ideally proposing a specific API.

Current API allows deleting providers only by `providerId `(ex. "google.com") which is not enough as there can be multiple email addresses sharing the same `providerId`. For example a user may have email addresses a@gmail.com, b@gmail.com and c@gmail.com linked to the same account and currently there is no way to delete the unnecessary email addresses.

As a related issue, it is unclear how `User.unlink` currently works if there are multiple email addresses with the same `providerId`.

I suggest either adding a new optional parameter `email` to the current API (`User.unlink`) or add a new API something like `User.unlinkWithEmail(String email)` or `User.removeEmail(String email)`.

## How would you use it?

I want to allow a user to update his/her email address to another one using the API `verifyBeforeUpdateEmail`. It is confusing if the old email address continues to work as before even if the user has updated the email address.

Currently if you use the API `verifyBeforeUpdateEmail`, the old email address is not removed but it stays as an additional authentication provider (actually not sure if this is a planned behavior or a bug). I would like to delete the old email address from the providers automatically after the `verifyBeforeUpdateEmail` has been successfully completed.

In addition I would like to display in the "Edit profile" page of my app the list of the linked email addresses and allow the user to delete any of them.