gguf-py : simplify support for quant types (#8838)

* gguf-py : use classes for quants

* convert_hf : simplify internal quantization type selection

* gguf-py : fix flake8 lint

* gguf-py : fix BF16 numpy view type

* gguf-py : remove LlamaFileTypeMap

Too specific to 'llama.cpp', and would be a maintenance burden
to keep up to date.

* gguf-py : add generic quantize and dequantize functions

The quant classes no longer need to be known,
only the target or the source type,
for 'quantize' and 'dequantize', respectively.