Adds hydra_defaults to builds and make_config

[rsokl]

Closes #253

There is now formal support for specifying a Defaults List via builds and make_config:

cs = ConfigStore.instance()

config_store.store(group="x", name="a", node=builds(int, 10))
Conf = builds(dict, x=None, y="hi", hydra_defaults=["_self_", {"x": "a"}])
job = launch(Conf, instantiate)

print(job.return_value)
# prints: {"x": 10, "y": "hi"}

Previously, one needed to specify defaults as a meta-field in builds, which was more complex for the user and which made the resulting yaml more verbose.

For make_config, one can specify defaults instead of hydra_defaults -- for the sake of parity with how a structured config is written -- to the same effect. Specifying defaults and hydra_defaults simultaneously raises an error.

Validation performed on hydra_defaults

Basic runtime type-checking is performed on hydra_defaults:

>>> make_config(hydra_defaults="not a list")
HydraZenValidationError: `hydra_defaults` must be type `list[str | dict[str, str | list[str]]`, Got: 'not a list'

>>> make_config(hydra_defaults=[{"a": 1}])
HydraZenValidationError: `hydra_defaults` must be type `list[str | dict[str, str | list[str]]`, Got: [{'a': 1}]

Static type checkers will also flag bad defaults lists:

We also make some attempt to do some basic value validation as well. Specifically, we raise on duplicate "_self_" entries:

>>> make_config(hydra_defaults=["_self_", "_self_"])
HydraZenValidationError: `hydra_defaults` cannot have more than one '_self_' entry

And we warn users when "_self_" is not present:

>>> make_config(hydra_defaults=[])
UserWarning: Defaults list is missing `_self_`. See https://hydra.cc/docs/upgrades/1.0_to_1.1/default_composition_order for more information
types.Config

Presently, hydra_defaults is overly permissive; it would be nice to provide stricter value-validation on its contents.