Allow OmegaConf.update to ignore struct flag and structured configs and add anyway

[omry]

Motivation:
In facebookresearch/hydra#1515, an attempt to use +foo.bar=10 (+ indicates forced addition in Hydra) is failing if foo is Structured Config.

The current implementation in Hydra is using open_dict on the config root, but Structured Configs are considered closed by default which means the flag override is not effective.
To address this, I will add a force_add flag to OmegaConf.update(), that will override the struct node directly on the manipulated node as it traverses the provided path.