Abstract config sources to allow additional configuration sources

[omry]

Currently Hydra supports absolute path (file system) and pkg:// (pkg_resource) for loading config files, but the implementation is a bit of a zoo.
Create an interface for ConfigSource, possible methods (inspired by pkg_resource):

class ConfigSource:
    def resource_stream()
    def resource_exists()
    def resource_listdir()
    def resource_isdir()

There is also the possibility that pkg_resource is extensible enough and can be used somehow.

Once this is done, it opens up the possibility of additional config sources, provided that they can implement the minimal required interface.

Finally, it should be possible to extend the supported configuration sources via a plugin.

[omry]

This can be used as base to support config in code at some point.

Initial idea:

hydra.experimental.HydraConfig.add("config_group", "config_name", OmegaConf.create({"blah": 10}))

and later config_group=config_name might be able to be used if HydraConfig is added to the search path.