Feat: add async VCF streaming scanner

[wjixiang]

Add async streaming support for VCF files via a new AsyncScanner trait.

Changes

• Add AsyncScanner trait (async_scanner.rs) with a stream-based
  scan() method
• Implement async scan on the existing Scanner struct using
  futures::try_unfold
• Move VcfModelBuilder to variant::model as ModelBuilder
• Add async test coverage (tokiotest)
• Remove unused generic Select<R>

Example usage

let scanner = Scanner::with_model(header, model);
let stream = scanner.scan(async_reader, Some(1024), None);
while let Some(batch) = stream.next().await {
    // process RecordBatch
}

Future work

• Async scan_query (requires AsyncSeek)
• Async scan_byte_ranges / scan_virtual_ranges