Refactor GKECreateClusterOperator's body validation

[moiseenkov]

Refactored GKECreateClusterOperator's body validation because it is really hard to understand current implementation:

1. Separated validation logic from the field value retrieval logic by moving out the latter into a separate method:

def _body_field(self, field_name: str, default_value: Any = None) -> Any:
    """Extracts the value of the given field name."""
    if isinstance(self.body, dict):
        return self.body.get(field_name, default_value)
    else:
        return getattr(self.body, field_name, default_value)

2. Separated validation of the body fields from each other.
3. Added warning messages when deprecated fields are used.
4. Current behavior of the operator wasn't changed.

[potiuk]

LGTM. Pending tests passing. Nice small refactor.