Have a max app name length and allowed characters check

[jhsinger-klotho]

as we sanitize resource names for IaC it will be a lot simpler if we have restrictions on the appname since we are going to use it in every resource name

[ghost]

Do we need to push this all the way to the customer, or can we do some sort of early-stage transform on it internally?

[ghost]

Discussed in triage. We're going to do this as written: it's an easy impl, and it's always easier to relax constraints than add new ones (from a backwards compat perspective).