Add list_adapters() to search for adapters

[calpt]

This PR includes two changes:

• adding list_adapters() to return a list with info on all adapters on AdapterHub ("ah") or the HF Hub ("hf")
• adding the possibility to load adapters directly via the file name, e.g. model.load_adapter("@ukp/distilbert-base-uncased_sentiment_sst-2_pfeiffer/") for https://adapterhub.ml/adapters/ukp/distilbert-base-uncased_sentiment_sst-2_pfeiffer/

In theory, something like this should now be possible to load any adapter from any source:

from transformers import AutoModelWithHeads, list_adapters

adapter_infos = list_adapters(source="ah")  # "ah", "hf" or None (for both)

model = AutoModelWithHeads.from_pretrained(adapter_infos[0].model_name)
model.load_adapter(adapter_infos[0].adapter_id, source=adapter_infos[0].source)

Using huggingface_hub changes from huggingface/huggingface_hub#152.

cc @Aaronsom

[hSterz]

Looks good