As per github.com/chupman request:

I was hoping to add the option of enabling login support via the mail field in addition to the login_attr field. To do this without hard coding the
attribute the config class would need to also expose the mail field from the foreman ldap configuration.

Use Case:
If a directory service uses serial numbers for employee uid and groupOfUniqueNames mappings, but users use an email for logins I believe it is current impossible to configure ldap with both group support and the ability to login with the user preferred attribute.