OmegaConf.to_object handling of init=False fields

[Jasha10]

This PR improves OmegaConf.to_object so that structured configs with init=False fields can be handled.

This comes up for example when calling to_object on a structured config generated by hydra_zen.builds (as such structured configs use an init=False field for the _target_ key).

Here is a demo of the use-cases enabled by this PR:

# example.py
from dataclasses import dataclass, field
from omegaconf import DictConfig, OmegaConf

@dataclass
class Foo:
    x: str = field(init=False)
    y: str = field(init=False, default="default")
    z: str = field(init=False)
    def __post_init__(self) -> None:
        self.z = "set_by_post_init"


cfg1 = OmegaConf.structured(Foo)
assert OmegaConf.is_missing(cfg1, "x")
assert cfg1.y == "default"
assert OmegaConf.is_missing(cfg1, "z")

obj1 = OmegaConf.to_object(cfg1)
assert not hasattr(obj1, "x")
assert obj1.y == "default"
assert obj1.z == "set_by_post_init"


cfg2 = OmegaConf.structured(Foo)
cfg2.x = "new_x"
cfg2.y = "new_y"
cfg2.z = "new_z"

obj2 = OmegaConf.to_object(cfg2)
assert obj2.x == "new_x"
assert obj2.y == "new_y"
assert obj2.z == "new_z"

Pre-PR behavior:

$ python example.py
Traceback (most recent call last):
  File "/home/rig1/dev/omegaconf/example.py", line 22, in <module>
    obj1 = OmegaConf.to_object(cfg1)
...
omegaconf.errors.MissingMandatoryValue: Structured config of type `Foo` has missing mandatory value: x
    full_key: x
    object_type=Foo

Post-PR behavior:

$ python example.py  # no errors, assertions succeed

Closes #789.

[pixelb]

looks good. thanks!