feat: add Content-Length Header when possible

[lhns]

Is it possible to know the object size before downloading?
This would allow showing a detailed progress bar for example.
If so it could be added as the Content-Length header to the response object.

[SgtPooki]

There are some cases in which we may know the content length, and we should add the header in those cases IMHO, but it's not currently specified in the trustless gateway specification, which we're targeting first with @helia/verified-fetch.

There are still some unknowns and things to clarify here, but we might be able to add a content-length header in some cases.

Relevant specs & issues:

• https://specs.ipfs.tech/http-gateways/path-gateway/#content-length-response-header
• Add Ipfs-DagSize and/or Ipfs-DataSize to gateway responses specs#461
• https://specs.ipfs.tech/http-gateways/trustless-gateway/ (no spec for content length)

---

For now, you may be able to use HTTP Range request headers to figure out more about the size of the content you're fetching.

E.g. Range: bytes=0-

And then you would get back a content-range header in the response with the size (if we can determine it): Content-Range: <unit> <range-start>-<range-end>/<size>

The logic for byte range handling is in https://github.com/ipfs/helia-verified-fetch/blob/main/packages/verified-fetch/src/utils/byte-range-context.ts

[lhns]

This works very well for me, thank you! I also found out that the onProgress event tells me the file size.