Better typing support

[acampove]

I think the typing used in this project could be improved. The way it is currently, it causes troubles to users. For example to_container returns a very broad type and the user has to cast the output to make sure the code does not throw errors.

    @staticmethod
    def to_container(
        cfg: Any,
        *,
        resolve: bool = False,
        throw_on_missing: bool = False,
        enum_to_str: bool = False,
        structured_config_mode: SCMode = SCMode.DICT,
    ) -> Union[Dict[DictKeyType, Any], List[Any], None, str, Any]:

Also, I would definitely not make cfg an Any. It must be something, a list, dictionary, something.

Describe the solution you'd like

In cases like this I would use overloads to make sure that the types going in and going out are under control and unambiguous.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

[acampove]

Another example of this issue can be seen below:

    cons = OmegaConf.merge(cons_1, cons_2)

where all the objects are meant to be DictConfig but the type analyzer says:

Type "ListConfig | DictConfig" is not assignable to return type "DictConfig"    Type "ListConfig | DictConfig" is not assignable to type "DictConfig" 

which stems from the the definition:

    @staticmethod
    def merge(
        *configs: Union[
            DictConfig,
            ListConfig,
            Dict[DictKeyType, Any],
            List[Any],
            Tuple[Any, ...],
            Any,
        ],
    ) -> Union[ListConfig, DictConfig]:
        """
        Merge a list of previously created configs into a single one

        :param configs: Input configs
        :return: the merged config object.
        """

Where overloading should definitely be used.