Add S3 storage backend for external storage

Implements ExternalStorage using @aws-sdk/client-s3 with pre-signed
GET URLs via @aws-sdk/s3-request-presigner. Exposed as a separate
entry point (@query-farm/vgi-rpc/s3) so the AWS SDK is only loaded
when used.

Features:
- Upload with PutObject + pre-signed GET URL generation
- Configurable bucket, prefix, region, endpoint (MinIO/LocalStack)
- zstd file extension (.arrow.zst) when content encoding is set
- Lazy SDK import on first upload

Usage:
  import { createS3Storage } from "@query-farm/vgi-rpc/s3";
  const storage = createS3Storage({ bucket: "my-bucket" });

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: rustyconover