[5.4] Use custom auth identifier column for "getAuthIdentifier" if set.

[DFurnes]

The Authenticatable trait includes methods to get the auth identifier name and value, but the getAuthIdentifier function will always return the primary key even if the column name is overridden.

This pull request updates that function to return the value for a custom auth identifier column, like the retrieveById and retrieveByToken methods in the EloquentUserProvider.