OmegaConf.structured(...) may mutate its input

[Jasha10]

Describe the bug
OmegaConf.structured(...) mutates its input when called on a dataclass or dataclass instance that has a structured DictConfig instance as one of its fields.

To Reproduce

from dataclasses import dataclass
from omegaconf import OmegaConf

@dataclass
class User:
    name: str = "Bond"
    age: int = 7

@dataclass
class HasUser:
    user: User

has_user1 = OmegaConf.structured(HasUser(User()))
assert has_user1.user._parent is has_user1
has_user2 = OmegaConf.structured(HasUser(has_user1.user))
assert has_user2.user._parent is has_user2
assert has_user1.user._parent is None  # has_user1.user has been mutated
assert has_user1.user is not has_user2.user

Expected behavior
The creation of has_user2 should not mutate has_user1.user.
Most OmegaConf creation and assignment methods do not mutate their inputs.

Additional context

• OmegaConf version: latest dev version (main branch)