Implement `get_evaluation_module_config_names()` function

In the `datasets` library, there is a handy `get_dataset_config_names()` function ([docs](https://huggingface.co/docs/datasets/v2.2.1/en/package_reference/loading_methods#datasets.get_dataset_config_names)) that returns all the available config names for a given dataset.

It would be useful to have a similar function in `evaluate`, since metrics like `glue` and `bleurt` have multiple config names, and programatic access to these options is handy.

However, a quick look at the `evaluate` codebase shows this isn't quite as easy to achieve as in `datasets`. The main limitation seems to be the absence of a builder config for metrics / evaluation modules. This is used in `datasets` [here](https://github.com/huggingface/datasets/blob/747f0c8612d4929dbdd1c72cca201815911ce660/src/datasets/inspect.py#L224) to quickly inspect the set of possible config names.

cc @TristanThrush this would allow us to avoid needing to import every metric explicitly in the evaluation service UI (when we check for supported metrics).