feat: add GGUFHeader::read

[McPatate]

Hello!

I needed a way to only read headers of .gguf files for performance reasons.

So I forked the repo and added what I needed to make it work.

cf https://github.com/McPatate/gguf-jinja-analysis if you're curious of what I used it for 😉

This PR does:

• add GGUFHeader::read as a public method
• add Clone to GGUFMetadataArrayValue & GGUFMetadataValue
• re-order dependencies in Cargo.toml (sorry OCD, can revert if you don't care for the change)

You can disregard this PR if it's not of any use.

[McPatate]

FYI, out of the 116k .gguf files I analysed, about 300-350 files' header failed to be parsed successfully. My intuition is that it is because some files have the .gguf extension when in fact they aren't, but there may be legit cases where the parser simply fails. Lmk if you want me to take a second look and gather more info on this.