feat: Add GenBank file format support for biological sequence data

[behroozazarkhalili]

Summary

Add native support for loading GenBank (.gb, .gbk, .genbank) files, a standard format for biological sequence data with annotations maintained by NCBI.

Changes

• Add genbank packaged module with pure Python state machine parser
• Register GenBank extensions in _PACKAGED_DATASETS_MODULES and _EXTENSION_TO_MODULE
• Add comprehensive test suite (28 tests)

Features

• Metadata parsing: LOCUS, DEFINITION, ACCESSION, VERSION, KEYWORDS, ORGANISM, taxonomy
• Feature parsing: Structured JSON output with location parsing (complement, join)
• Sequence parsing: ORIGIN section with automatic length calculation
• Compression support: gzip, bz2, xz via magic bytes detection
• Memory efficiency: Dual-threshold batching (batch_size + max_batch_bytes)
• Large sequences: Uses large_string Arrow type for sequences/features

Usage

from datasets import load_dataset

# Load GenBank files
ds = load_dataset("genbank", data_files="sequences.gb")

# With options
ds = load_dataset("genbank", data_files="*.gbk", 
                  columns=["sequence", "organism", "features"],
                  parse_features=True)

Test plan

• All 28 unit tests pass
• Tests cover: basic loading, multi-record, compression, feature parsing, column filtering, batching, schema types