Which issue does this PR close?

• Closes feat: expose registerObjectStore for S3 / GCS / HTTP backends #70 .

Rationale for this change

SessionContext.registerParquet(name, "s3://bucket/...") (and the read/register counterparts for CSV / NDJSON / Arrow / Avro) accept arbitrary path strings, but until this PR there was no Java surface to attach an object_store::ObjectStore to a URL scheme + bucket. As a result, s3://, gs://, and remote https:// paths fail with "No suitable object store found for ..." unless the embedder shells out to process-level environment variables — and even then, multi-tenant JVMs cannot give two contexts different credentials in the same process.

DataFusion's Rust RuntimeEnv::register_object_store(url, store) already solves this end of the problem; the gap was purely in the Java surface above the JNI line. This PR closes it with a typed, per-context registration API:

SessionContext ctx = SessionContext.builder()
    .registerObjectStore(ObjectStoreOptions.s3()
        .bucket("my-bucket").region("us-east-1")
        .accessKeyId("...").secretAccessKey("...").build())
    .registerObjectStore(ObjectStoreOptions.s3()
        .bucket("other").endpoint("https://minio.internal:9000")
        .allowHttp(true).build())
    .build();

ctx.registerParquet("orders", "s3://my-bucket/orders/");

What changes are included in this PR?

• Proto: new proto/object_store_options.proto
• Java API: new org.apache.datafusion.ObjectStoreOptions
• Native: new native/src/object_store.rs
• Cargo features: object-store-{aws,gcp,http}, with default = ["object-store-aws", "object-store-gcp", "object-store-http"]
• Build wiring: proto/object_store_options.proto

Are these changes tested?

23 new tests across ObjectStoreOptionsTest and SessionContextObjectStoreTest

Are there any user-facing changes?

Yes, additive only, no breaking changes:

• New public class org.apache.datafusion.ObjectStoreOptions (sealed) with three concrete subtypes S3 / Gcs / Http and per-backend builders. Static factories: ObjectStoreOptions.s3(), .gcs(), .http(listingUrl).
• New public method SessionContextBuilder.registerObjectStore(ObjectStoreOptions).
• Generated protobuf classes ObjectStoreRegistration, S3Options, GcsOptions, HttpOptions under org.apache.datafusion.protobuf (consistent with existing options bundles).
• New repeated ObjectStoreRegistration object_stores = 8 field on SessionOptions (proto-3 forward compatible — older builds simply ignore it).

Existing APIs are unchanged. The make build picks up object-store-{aws,gcp,http} Cargo features by default; downstream Rust builds that strip a feature trip a clear runtime error if a caller registers the missing backend.