Add AttributeConsmingService metadata.

With the changes made, the following XML is added to metadata requests:

<md:AttributeConsumingService index="1">
   <md:ServiceName xml:lang="en">Drupal Documentation</md:ServiceName>
   <md:RequestedAttribute Name="username" FriendlyName="User name" isRequired="true"/>
   <md:RequestedAttribute Name="email" FriendlyName="User email" isRequired="true"/>
</md:AttributeConsumingService>

An attribute is added for each user name and email attribute specified in the login configuration. It the attribute is specified in configuration, it is also marked as required since the expectation is it will exist.

Closes #3548523