GCSFileIO: use single-shot upload for small objects below a configurable threshold

[manideep4597]

Feature Request / Improvement

Summary

GCSFileIO / GCSOutputStream always uploads via GCS WriteChannel (storage.writer(...)), which uses resumable uploads. For small objects this adds unnecessary round-trips and latency compared to a single-shot / simple upload (storage.create(...)).

S3FileIO already switches between single PutObject and multipart based on size (s3.multipart.part-size-bytes, s3.multipart.threshold). GCS has no equivalent.

Motivation / Use case

Iceberg writes many relatively small objects (manifests, metadata JSON, delete files, etc.). For these, resumable-upload setup cost often dominates transfer time.

Google Cloud Storage Java guidance also recommends:

• small content → storage.create(blobInfo, bytes) (single request)
• large / streaming content → storage.writer(...) (resumable)

Today Iceberg always takes the resumable path, which is suboptimal for small files and creates a feature gap vs S3.

Current behavior

In GCSOutputStream, uploads always open a write channel:

WriteChannel channel =
    storage.writer(
        BlobInfo.newBuilder(blobId).build(), writeOptions.toArray(new BlobWriteOption[0]));
gcpProperties.channelWriteChunkSize().ifPresent(channel::setChunkSize);
stream = Channels.newOutputStream(channel);

### Query engine

_No response_

### Willingness to contribute

- [ ] I can contribute this improvement/feature independently
- [x] I would be willing to contribute this improvement/feature with guidance from the Iceberg community
- [ ] I cannot contribute this improvement/feature at this time