Add new resolvers oc.dict.keys and oc.dict.values

[odelalleau]

Description

Some config options that are stored as a DictConfig may sometimes be easier to manipulate as lists, when we care only about the keys or the associated values.

Adding new resolvers oc.dict.keys and oc.dict.values will make it easy to access either keys or values as a list:

cfg = OmegaConf.create("""
machines:
  node007: 192.168.0.27
  node012: 192.168.0.35
  node075: 192.168.1.89
nodes: ${oc.dict.keys:${machines}}
ips: ${oc.dict.values:${machines}}  
""")

print(f"cfg.nodes: {cfg.nodes}")  # cfg.nodes: ['node007', 'node012', 'node075']
print(f"cfg.ips: {cfg.ips}")      # cfg.ips  : ['192.168.0.27', '192.168.0.35', '192.168.1.89']

Additional context
See related discussion in #578 (comment)