deprecate exclude_structured_configs; introduce SCMode enum

[Jasha10]

exclude_structured_configs is new in 2.1 and we can still change it.
Looks like an enum can replace exclude_structured_configs and instantiate:

class SCMode(Enum):
  DICT = 1            # convert to plain dict
  DICT_CONFIG = 2     # Keep as OmegaConf DictConfig
  INSTANTIATE = 3     # instantiate underlying object.

Originally posted by @omry in #502 (comment)

Based on the above, it is planned to deprecate the exclude_structured_configs argument to the OmegaConf.to_container function. In it's place, it is proposed to add a structured_config_mode: SCMode argument to the to_container function's signature. This argument will determine what behavior occurs when to_container encounters a StructuredConfig.

To preserve the current default behavior, the default value will be structured_config_mode = SCMode.DICT.

[Jasha10]

The SCMode.INSTANTIATE mode is under development in PR #502

[Jasha10]

Closing as duplicate of #556