Add option to register structured config by name

[soldni]

It would convenient to be able to register structured config by name, so they can be easily recalled via a resolver.

Imagine something like this:

@sp.nickname("ducky")      # this is new 
@dataclass
class DuckyConfig:
    height: float: 3.
    color: str = 'yellow'

and then in a config:

root:
    toy: ${sp.nickname:ducky}     # this looks up "ducky" and places it here

Might have to figure out how to customize nicknamed configs on use.