add help text and validation customer create

[xavpaice]

• Updates the help text for replicated customer create to be clear that --email is required if --helm-install is set

The error if someone missed --email is clear from the API response, no need to add extra validation here:

replicated customer create --app xav-pottery-shop --name "testy test test" --channel Stable --type trial --helm-install 
ERROR: 400
METHOD: POST
ENDPOINT: https://api.replicated.com/vendor/v3/customer
email is required for customers with helm install enabled
Error: create customer: create customer: POST https://api.replicated.com/vendor/v3/customer 400: email is required for customers with helm install enabled