feature: Add protected FindParent method

[gtg092x]

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This will allow implementors the opportunity to override runtime Find behavior. This is particularly useful for projects that implement multiple potential parent scopes and need a way to disambiguate them.

For example, some child scopes might want to specifically find ancestors up their transform tree:

public class ParentedLifetimeScope : LifetimeScope {
    protected override LifetimeScope FindParent(Type type) {
        LifetimeScope[] allScopes = FindObjectsByType(type, FindObjectsSortMode.None).Cast<LifetimeScope>().ToArray();
        return allScopes.First(x => IsAncestor(x.gameObject));
    }

    public bool IsAncestor(GameObject target) {
            var parent = this.transform.parent;
            while (parent != null)
            {
                if (parent == target.transform)
                {
                    return true;
                }

                parent = parent.parent;
            }

            return false;
        } 
}

All unit tests pass with this change and because the implementation is a virtual method that defaults to the previous functionality, current behavior is unchanged.