Add Ipfs-DagSize and/or Ipfs-DataSize to gateway responses

This is a placeholder issue for writing spec and tracking implementation.

The need is to expose equivalent of `ipfs files stat`'s `Size` and `CumulativeSize` on HTTP gateway somehow, allowing light clients to understand how big is the DAG before fetching it as a CAR etc.

* `Ipfs-DagSize` total size of the DAG (user data + DAG metadata, IPLD envelopes etc)
* `Ipfs-DataSize` total size of the raw data (without DAG metadata, IPLD envelopes etc)

We don't want expensive equivalent of `ipfs dag stat` for Unixfs, we want to leverage hints from UnixFS. It could be limited to formats like UnixFS, or single-block `dag-cbor` etc, which include total sizes in the root blocks, removing the need for expensive `ipfs dag stat` of entire DAG.

### Implementation options

- :green_circle: HTTP Headers
- `Ipfs-DagSize` and `Ipfs-DataSize` headers with value in bytes
- pro: simple and easy to implement, JS apps already use `Content-Type` for this (because without compression it matches payload size), we should not break them without giving a replacement header
- 💡 caveat: unnecessary work on every request, unless we limit them to `HEAD` responses
- :no_entry: Custom content type that returns useful metadata
- would be opt-in `Accept` HTTP header or `format` URL arg)
- pro: could be leveraged for things other than size info
- con: complexity, need to decide on JSON schema, harder for gateways and clients to adopt

cc @vasco-santos I know you needed something like this a while ago