Add Execution Extensions (ExEx) support for custom indexing and event hooks

## Summary

Enable Reth's Execution Extensions (ExEx) framework in ev-reth so customers can build custom indexers, analytics pipelines, bridge monitors, and event-driven integrations as in-process post-execution hooks.

## Motivation

Currently there is no built-in mechanism for customers to react to block execution events (new blocks, reorgs, state changes) without polling JSON-RPC. ExExes run in the same process as the node using shared memory, delivering 10-100x better performance than RPC-based indexers. They are reorg-safe by design via `ChainCommitted`, `ChainReorged`, and `ChainReverted` notifications.

**Customer use cases:**
- Block explorers and transaction indexers
- Analytics and reporting pipelines
- Bridge monitors and webhook systems
- Custom event processors for Evolve-specific transaction types (0x76)

## Scope

1. Wire up the `install_exex` API on the node builder in `bin/ev-reth/src/main.rs`
2. Ensure ExEx notifications correctly include `EvTxEnvelope` (0x76) transactions, batch calls, and sponsorship metadata
3. Provide at least one example ExEx (e.g., a simple block logger or Evolve tx indexer) in an `examples/` directory
4. Document how customers can write and compile custom ExExes against ev-reth
5. Evaluate Remote ExEx support for out-of-process consumers (gRPC)

## References

- [Reth ExEx Overview](https://reth.rs/exex/overview/)
- [Paradigm: Execution Extensions](https://www.paradigm.xyz/2024/05/reth-exex)
- [ExEx Examples](https://github.com/paradigmxyz/reth-exex-examples)
- [Community ExEx Directory](https://www.exex.rs/)