feat: add fb benchmark tests

[brendontan03]

Description

Add benchmark tests for flashblock subscription with new headers and new transactions.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Other (please describe):

Code Guidelines

Before submitting your PR, please review the relevant code guidelines in the docs/ folder:

• Building on Reth Guide - Comprehensive guide for building on top of Reth

Specific Guidelines by Component:

• Chainspec & EVM Guide - For changes to chainspec or EVM configuration
• Database Guide - For changes to database tables or storage
• Engine & Consensus Guide - For changes to consensus or engine API
• Extension Guide - For adding extensions or plugins
• Node Builder Guide - For changes to node initialization
• Payload Builder Guide - For changes to block building logic

Checklist

• I have reviewed the relevant code guidelines in the docs/ folder
• My code follows the coding standards of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Testing

To run tests individually:

cargo test -p xlayer-e2e-test --test flashblocks_tests -- fb_benchmark_new_heads_subscription_test --include-ignored --nocapture --test-threads=1

cargo test -p xlayer-e2e-test --test flashblocks_tests -- fb_benchmark_new_transactions_subscription_test --include-ignored --nocapture --test-threads=1

To run entire flashblock test suite:

cargo test -p xlayer-e2e-test --test flashblocks_tests -- --include-ignored --nocapture --test-threads=1

Additional Notes

• new headers test screenshot

• new transactions test screenshot