This feature is subtle but very important: Currently, when a user logs in via an "external" auth system (such as Github), they are either logged into an existing bound account or a new account is created for them in the database. While this normally works jut fine, it hits a roadblock when the *internal* auth system configured is not the database, but instead something like LDAP. In that case, *most* Enterprise customers will prefer that logging in via external auth (like OIDC) will also *automatically* bind the newly created account to the backing *internal* auth account. For example, login via PingFederate OIDC (backed by LDAP) should also bind the new QE account to the associated LDAP account, via either username or email. This change allows for this binding field to be specified, and thereafter will perform the proper lookups and bindings.
Moves all the external login services into a set of classes that share as much code as possible. These services are then registered on both the client and server, allowing us in the followup change to dynamically register new handlers
