Clarification is needed on what a valid `username` is

[timothydilbert]

All of the Zendesk documentation refers to the username being [email protected]/token. The ZendeskApiClient only wants the email portion.

To avoid confusion, you should either:

# Change from this:
IServiceCollection.AddZendeskClient(string endpointUri, string username, string token)

# To This:
IServiceCollection.AddZendeskClient(string endpointUri, string email_address, string token)

Or, you should put in validation that throws an Exception when the username variable ends with /token.