Enable OmegaConf.register_new_resolver usage as decorator

[Jasha10]

Currently use of register_new_resolver requires (1) defining a function and (2) calling register_new_resolver on it. Enabling register_new_resolver to function as a decorator could simplify this workflow:

...

@OmegaConf.register_new_resolver
def my_resolver(cfg):
    ...

# In this proposal, the above would be equivalent to the following:

def my_resolver(cfg):
    ...

OmegaConf.register_new_resolver("my_resolver", my_resolver)

[Northo]

I think this is a very good idea!