Get some info about UnixFS objects on public IPFS HTTP API

### Checklist

- [X] My issue is specific & actionable.
- [X] I am not suggesting a protocol enhancement.
- [X] I have searched on the [issue tracker](https://github.com/ipfs/go-ipfs/issues?q=is%3Aissue) for my issue.

### Description

I am implementing a backend to access IPFS via the Python library [fsspec](https://filesystem-spec.readthedocs.io) at [ipfsspec](https://github.com/d70-t/ipfsspec). To do so (and to save me from implementing the IPFS protocol in Python), the plan is to access UnixFS files and directories on IPFS via a HTTP gateway. An `fsspec` backend needs to implement a function `info(path)` which must return

* if the thing behind a path is a directory or a file
* if it is a file, the size of the file

To me, this seems to be a reasonable requirement for other generic filesystem abstractions as well, thus I assume that this feature request could be of broader interest.

While the [`/v0/files/stat`](https://docs.ipfs.io/reference/http/api/#api-v0-files-stat) provides this kind of information, this endpoint is often not reachable on public gateways.

Another option to obtain this information is to perform a `HEAD` request towards `http://gateway/ipfs/CID`, which in case of a file provides the size in the `content-length` header and which (seemingly) lets me discriminate between file and directory using the `etag` header. This method works on some public gateways, but scares me as well, as this doesn't seem to be the right use of observable API features.

I see three possible ways to obtain the desired functionality:
* It is already implemented and I didn't find it?
* Move / replicate the `files/stat` API to public gateway port (could this be `GET` as well?)
* Implement and document HTTP headers which include this information and are to be returned when `/ipfs/CID` is requested

Tagging @whyrusleeping as I've been talking to him about this already on slack.