Add hf_hub_download / snapshot_download as aliases in HfApi

[Wauplin]

Discussion triggered by @Narsil on slack (private link) cc @hysts. Let's keep the same naming to avoid confusion. A few args would be shared/defined at HfApi level (token, endpoint, user_agent,...). The main goal is to avoid passing those args all the time in a script (same as upload_file/upload_folder).

Implementation-wise, we need to implement endpoint parameter in hf_hub_download which is a long-requested feature anyway 😄 (#1082).