Expose a function to update the meta data in the readme

[lvwerra]

As discussed in huggingface/evaluate#6 we would like a function to update the meta data in the readme of a repo:

huggingface_hub.metadata_update(
  repo_id="lvwerra/my_model",
  update={"metrics": new_value}
)

This would enable to programmatically update meta data such as metrics from the evaluate, datasets, or transformers libraries.

There are two use-cases:

1. The key does not exist yet so we add an entry.
2. The key does already exist but needs to be updated - in that case we might want a overwrite=True option as safety.

What do you think @julien-c, @osanseviero? I can have a look at this if you want.

[julien-c]

sounds good to me!

this can use the existing metadata-related functions under the hood, and maybe rely on upload_file so it doesn't need a local install of git

[osanseviero]

Yes, I think using single file download + metadata_load/metadata_save (https://github.com/huggingface/huggingface_hub/blob/main/src/huggingface_hub/repocard.py) + upload_file would be very good idea