Scaffold a singleton type

[lumtis]

Scaffold a singleton type with the command starport type fields... --singleton or starport type fields... --unique where the type has only one instance in the store

This is used for global customizable variable for the app like the params.

I got the idea of it because it would be used with spn: the testnet monitoring module must store a structure named "Consensus State" of spn. This is a single instance of structure that must be stored in the store

This type would have the --no-message flag as well. To keep consistent with the CRUD format we would have

• create: create the type and fails if it already exists
• read: query the type, takes no param
• update: update by providing a new value, fails if it doesn't exists
• delete: delete the type, fails if it doesn't exist

Internally, the keeper would simply expose the methods SetFoo, GetFoo and RemoveFoo

We would then have a nice representation of the set of features from the type command:
starport type --singleton generates a variable
starport type generates an array
starport type --indexed generates a map

[lumtis]

This is low priority.

If you use it internally you can just scaffold an indexed type and use a constant for the index.