Use LazyLoadingBackend as default backend

[dimartiro]

Motivation

Add initial forking support by introducing a lazy-loading backend that retrieves state from the forked chain on demand.

Solution

Replace Substrate’s default backend with the new ForkedLazyBackend, which is composed of:

1. ForkedLazyBackend

• Core backend implementing the sp_state_machine::Backend trait
• Lazily fetches storage values from the forked network via RPC
• Maintains an in-memory cache (db) of previously accessed state
• Tracks removed keys to prevent unnecessary remote fetches

2. RPCClient

• Abstraction layer for RPC communication with the forked network
• Provides methods to fetch blocks, headers, storage values, storage hashes, and paginated keys

3. Blockchain

• In-memory blockchain storage for cached blocks and headers
• Lazily loads block data via RPC when not available locally
• Maintains fork metadata (best block, finalized block, leaves)

4. RawIter

• Custom storage iterator that merges local and remote state
• Iterates over storage keys by checking the local cache first, then falling back to RPC
• Respects prefix filters and removed keys during iteration

---

Note: Basic tests were also added to verify the lazy-loading behavior.

---

Closes: ChainSafe/gossamer-parity#59, ChainSafe/gossamer-parity#61, ChainSafe/gossamer-parity#62, ChainSafe/gossamer-parity#63, ChainSafe/gossamer-parity#65

[timwu20]

Can you fix the CI checks?