gguf: Add type definitions for split.* metadata + sanity check

[ngxson]

This PR add correct type definitions for GGUF split.*, and use them in ggufAllShards for sanity checks.

The sanity checks does not really serves real-life purpose, since it is very unlikely that users mess up with file name. For now, it mostly to demonstrate how to use these metadata keys.

This PR also modify output format of ggufAllShards. Because metadata is saved in the first split only, we can simply discard metadata of other splits.

For more details, please have a look on gguf-split source code:

https://github.com/ggerganov/llama.cpp/blob/24ecb58168dce81646c2ed425690a106591c8c6d/examples/gguf-split/gguf-split.cpp#L223-L225

[julien-c]

maybe of interest to @phymbert too

[julien-c]

and btw related a bit to ggml-org/ggml#826

[coyotte508]

what's the status of this? (btw feel free to merge PRs in huggingface/gguf)