api: rethink user identity

This feels a bit out of place here.

The problem was that to publish a record, we needed the user's identity. Therefore, we used a utility function from invenio-access to generate this identity given the user id.

But, there is permission needed to publish a draft (`authenticated_user`) and it was added here.

Possible alternatives:

1 - move this to `invenio-rdm-records` as this is a specific RDM action (publish)
2 - move this to `invenio-access` as a new utility function (e.g. `get_authenticated_identity`)

_Originally posted by @alejandromumo in https://github.com/inveniosoftware/invenio-github/pull/99#discussion_r1250437186_