Gguf dump start data offset via --data-offset and some extra refactor (#8054)

* gguf-dump: add --data-offset

* gguf-dump: add tensor data offset table

* gguf-dump: refactor GGUFReader for clarity

* gguf-dump: add --data-alignment

* gguf-dump.py: Rename variables and adjust comments

start_data_offset --> data_offset

_build_tensors_info_fields --> _build_tensor_info