Add support for GGUF machine learning model files

[fegge]

This PR adds support for the GGUF file format. GGUF is used to store machine learning models for executors based on the GGML machine learning library like llama.cpp and whisper.cpp developed by Meta.

Since there was no obvious location to add machine-learning model file formats, and since there are multiple other ML specific files formats that could be added in the future, I chose to create a new top-level ml directory and added the gguf.ksy file under that directory. I'm happy to put it somewhere else if someone has opinions on the location.

Example files for testing can be found under the models directory in the llama.cpp repository. (Small model files can be generated using the gguf utility in llama.cpp.)

[armijnhemel]

I guess this should be changed to https://github.com/ggml-org/ggml/blob/master/docs/gguf.md ?

[generalmimon]

@armijnhemel:

I guess this should be changed to https://github.com/ggml-org/ggml/blob/master/docs/gguf.md ?

I agree, but please not master - always pin URLs to commit hashes to make them permanent. Pointing to the master branch doesn't even make sense semantically - this .ksy specification follows a particular current version of that Markdown file, not whatever version appears there in the future, which may be very different.

I should mention this rule in CONTRIBUTING.md and https://doc.kaitai.io/ksy_style_guide.html when I have some time...

[armijnhemel]

This offset could be used to see and check if the data is really present. I took a GGUF file and cut it, was able to parse it, but it didn't do an extra sanity check to see if the offset was actually valid.

@generalmimon would you agree?

[generalmimon]

@armijnhemel:

This offset could be used to see and check if the data is really present.

Sure, why not. I'd prefer an "implicit check" by actually parsing the data at that offset using instances if possible - if the offset points after the end of file, parsing will fail. An "explicit check" using valid is also an option.