Implement forking support with lazy loading backend

[dimartiro]

Description

This PR implements forking functionality for anvil-polkadot, allowing users to fork from a remote Substrate/Polkadot node and run local tests against live network state.

Adds the ability to fork from a running anvil-polkadot node (or compatible Substrate node) using the --fork-url flag. The implementation uses a lazy-loading approach to fetch state from the remote node on-demand, caching values locally for performance

Manual testing

After compiling the entire project using cargo build --release

1. Start base node

RUST_LOG=info ./target/release/anvil-polkadot --port 9970

2. Modify state on base node

./target/release/cast send 0x70997970C51812dc3A010C7d01b50e0d17dc79C8 \
    --value 5ether \
    --private-key 0xac0974bec39a17e36ba4a6b4d238ff944bacb478cbed5efcae784d7bf4f2ff80 \
    --rpc-url http://localhost:9970

3. Verify modified balance

./target/release/cast balance 0xf39Fd6e51aad88F6F4ce6aB8827279cffFb92266 \
    --rpc-url http://localhost:9970

Expected Output: ~9995 ETH (10000 - 5 - gas)

4. Start fork node (inherits modified state)

RUST_LOG=info ./target/release/anvil-polkadot \
    --port 8545 \
    --fork-url http://localhost:9944

5. Verify fork inherited state

./target/release/cast balance 0xf39Fd6e51aad88F6F4ce6aB8827279cffFb92266 \
    --rpc-url http://localhost:8545

Expected Output: ~9995 ETH ✅ (matches base node)

6. Make changes on fork (doesn't affect base)

./target/release/cast send 0x70997970C51812dc3A010C7d01b50e0d17dc79C8 \
    --value 1ether \
    --private-key 0xac0974bec39a17e36ba4a6b4d238ff944bacb478cbed5efcae784d7bf4f2ff80 \
    --rpc-url http://localhost:8545

7. Verify isolation

./target/release/cast balance 0xf39Fd6e51aad88F6F4ce6aB8827279cffFb92266 \
    --rpc-url http://localhost:9970

Exepected Output: ~9995 ETH (unchanged)

./target/release/cast balance 0xf39Fd6e51aad88F6F4ce6aB8827279cffFb92266 \
    --rpc-url http://localhost:8545

Expected Output: ~9994 ETH (changed)

Integration tests

• Check forked node retrieves right state
• Test forking using latest finalized block and block height (positive and negative numbers)
• Fork a node with a smart contract deployed and check storage is the same

Closes: ChainSafe/gossamer-parity#52, ChainSafe/gossamer-parity#58, ChainSafe/gossamer-parity#65, ChainSafe/gossamer-parity#84

[alindima]

Looking good. Let's bring in the latest changes from the feature branch

[alindima]

not neccessarily for this PR, but have a look at what other forking-related test scenarios regular anvil has. We should mirror them where it makes sense

[alindima]

as an example, what we discussed earlier in the call, using calls which mutate the state on top of a forked network (anvil_setBalance, etc)

[dimartiro]

But we are already doing that with test_fork_preserves_state_and_allows_modifications right?