Adds an option to enable sAMAccountname logins when upndomain is set

[kwagga]

Description

Active Directory allows LDAP binds as userprincipalname@upndomain as well as samaccountname@updomain.

With the current LDAP filter samaccountname logins fail when the upndomain configuration parameter is set, since the filter only only checks for userprincipalname=username@updomain.

This PR provides an enable_samaccountname_login option that can be set in the LDAP Authentication method. This will cause the LDAP user search filter to match either userprincipalname or samaccountname attributes instead of just the userprincipalname when the upndomain configuration parameter is set.

This PR relies on hashicorp/cap#146

TODO only if you're a HashiCorp employee

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