Add automatic list key generation (autokey)

There are many use-cases where the list key does not contain any semantics other than being a unique identifier.

In this mode, the designer does not want to define or implement the list key management.
The client does not want to provide a new unique list key by first locking the config and retrieving all existing entries to find an unused entry
The current protocol operations do not support this feature and need improvements
The key is a uint32 number from 1 .. max
The value 0 is reserved for indicate no key
Not allowed for ordered-by user, or no insert operation allowed for create
The autokey is un-named so it does not conflict with any siblings, or could use reserved name like "_id_"
This is not the same as position-based (no key-stmt at all) because autokey does not renumber for any reason
The server assigns an unused value for the ID and the protocol operation enhancements allow this operation and convey the assigned ID to the client
```
list shopping-list {
autokey;
// list child nodes
}
```

netmod-wg / yang-next