Client-side suggested defaults

Copied from https://github.com/Azure/typespec-azure-pr/issues/800

Per azure/typespec-azure-pr#728 We have a mechanism for providing service-side defaults, but need a mechanism that allows simplifying the client-side SDK by providing a suggested default value for required properties. This allows client SDKs to simplify an API, or prevent an SDK breaking change when there is a REST API breaking change making a formerly optional property required.

One proposal is to use a decorator @suggestedValue in cadl core that provides the value hint:

model SomeModel {
  @suggestedValue("Value that client SDKs may supply by default if they make this property optional") requiredThing: string;
}

Usages

Simple scalars

model Widget {
  @suggestedValue(1)
  count: int
}

Suggested client SKU

enum AccountType { Free, Basic, Advanced}
model WidgetAccount {
  @suggestedValue(AccountType.Basic) // AcountType.Basic::value??
  type: AccountType;
}

Default object or collection

model FirewallRule
{
   action: string;
   port: string;
   ipRange?: string;
}

model DefaultRule extends FirewallRule {
  action: "Deny";
  port: "*"
}

model AllowHttps extends FirewallRule {
  action: "Allow";
  port: 443;
}

model MyHttpsConnectedWidget {
  @suggestedValue([DefaultRule, AllowHttps])
  firewallRule: FirewallRule[];
  ...
}

As this applies to model properties, it would analogously apply to parameters

Questions / Considerations

checking will be a bit tricky - ideally, the decorator would enforce that the provided type is assignable to the decorated property type and that it consists entirely of defined literals (StringLiteral instead of string, for example)
Are we missing any scenarios using a decorator
If a decorator, what library does this decorator belong in?

Azure / typespec-azure