Add BackendReader trait

[sergerad]

Context

In #967 we created reader traits used by rocks db structs including a new snapshot reader struct.

We need to do the same for the large forest SMT traits and PersistentBackend.

Changes

• Extracts a BackendReader trait from Backend, providing the read-only query interface for the SMT forest storage backend (open, get, get_leaf, version, lineages, trees, entries, entry_count).
• Backend becomes a supertrait of BackendReader, extending it with write operations and an associated Reader type with a reader() method that returns a point-in-time snapshot.
• Adds InMemoryBackendSnapshot — a read-only, snapshot of the in-memory backend — implementing only BackendReader.
• Adds PersistentBackendReader — a read-only view of the persistent backend backed by a RocksDB snapshot, providing a consistent read view isolated from concurrent writes (Similar to RocksDbSnapshotStorage).