Add API to drop a specific snapshot by ID

[manuzhang]

ExpireSnapshots (src/iceberg/update/expire_snapshots.h) supports retention-driven expiration (oldest-than, max-count). It does not expose "drop exactly snapshot X" as a first-class op.

Use cases:

• Compaction / streaming ingestion that wants to invalidate a specific bad snapshot
• Recovery: after a failed write that produced a snapshot, drop it without waiting for retention
• Mirror pipelines (Flink CDC, lakehouse mirror jobs) that produce one snapshot per cycle and need surgical removal

API sketch

Result<std::shared_ptr<DropSnapshot>> Table::NewDropSnapshot();
DropSnapshot& DropSnapshot::SnapshotId(int64_t id);
// Refuses if id is referenced by a branch tip or tag; --force flag to override.

Could be implemented as a thin wrapper over ExpireSnapshots with expireSnapshotId(id).

[manuzhang]

Upstream status: tracked, not yet implemented.

• apache/iceberg-cpp#364 (OPEN, umbrella) lists ManageSnapshots (<- SnapshotManager) as unchecked, owner @zhjwpku.
• apache/iceberg-cpp#351 (CLOSED, not merged) attempted the ManageSnapshots interface — covered rollback / cherrypick / branches / tags / retention.
• ExpireSnapshots is merged (apache/iceberg-cpp#490) but does not expose expireSnapshotId(id).

Recommendation: keep this open. Two reframings worth filing on apache, in order of effort:

1. Small, additive: expose expireSnapshotId(id) on the existing ExpireSnapshots.
2. Land ManageSnapshots per umbrella [Umbrella] Implement PendingUpdate subclasses apache/iceberg-cpp#364.