feat(cloud-storage): add pluggable cloud storage support for distributed HugeGraph persistence

[vaijosh]

Why

This PR introduces a unified cloud storage capability for HugeGraph’s distributed stack, so store data can be synchronized to cloud object storage for stronger durability and recovery options.

The focus is not a provider-specific implementation detail, but a cloud-capable architecture with a default S3-compatible provider and extension points for additional providers.

What this PR delivers

1) Cloud storage support in distributed runtime

• Enables cloud sync for store-side RocksDB in distributed deployments (backend=hstore flow).
• Supports both:
  • cloud-first mode: commit waits for cloud sync (stronger durability)
  • async mode: local/Raft commit first, periodic cloud reconciliation

2) Pluggable provider architecture

• Introduces provider/client abstractions for cloud backends.
• Uses ServiceLoader discovery so new cloud providers can be added via plugin JARs.
• Keeps built-in s3 provider as default (S3-compatible API model), while allowing future providers without core rewrites.

3) Cloud-neutral configuration model

• Standardizes configuration naming around cloud-neutral keys (cloud_*).
• Aligns server-side propagation and store-side consumption of cloud settings.
• Cleans up older S3-specific naming to keep the config surface consistent and provider-agnostic.

4) Operational docs and examples

• Updates architecture and usage docs to describe cloud storage behavior consistently.
• Adds/updates sample plugin guidance and SPI wiring for provider developers.
• Aligns docker/dev scripts and templates with cloud storage configuration.

User Impact

• Resilience against Ephemeral Infrastructure: By treating cloud object storage as the decoupled, durable source of truth, HugeGraph can seamlessly adapt to cloud-native environments (e.g., Kubernetes). If an instance or pod is unexpectedly terminated, rescheduled, or suffers from local disk/EBS detachment, the system prevents catastrophic data loss. New instances can instantly rehydrate or recover their state directly from the cloud checkpoint.
• Flexible Durability SLAs: Operators running distributed deployments (backend=hstore) can choose their optimal trade-off between throughput and durability:
  • Cloud-First Mode (sync): Guarantees zero data loss by ensuring local Raft commits inline with cloud storage flushes before returning success to the caller.
  • Asynchronous Mode (async): Minimizes latency by performing background reconciliation to the cloud storage bucket within a bounded time horizon.

Validation

• Maven compile/build verification on affected modules.
• Manual Validation and smoke test validation check RocksDB-cloud.md for more details.

Reviewer focus areas

• Cloud config key consistency across server/store/docs
• Provider abstraction and ServiceLoader integration
• Cloud-first vs async durability semantics
• Recovery/rehydration behavior and operational clarity