feat(examples): add multi-transaction block simulation example

[danieljrc888]

Summary

• Adds a new multi_tx_block_sim example demonstrating how to simulate an entire block of transactions using REVM
• Shows two execution strategies: per-transaction commit (transact_commit) and batch commit (transact_many_commit)
• Covers contract deployment, multi-account interaction, value transfers, gas tracking, and balance reporting

What the example demonstrates

1. Configuring a realistic block environment (number, timestamp, coinbase, basefee)
2. Deploying a counter contract and having multiple accounts interact with it
3. Tracking cumulative gas usage across all transactions in a block
4. Comparing per-transaction commit vs batch commit strategies

Test plan

• Example compiles (cargo build -p example-multi-tx-block-sim)
• Example runs successfully (cargo run -p example-multi-tx-block-sim)
• CI passes