feat: add SLOAD/SSTORE mixed benchmarks from execution-spec-tests

[CPerezz]

Migrated benchmark tests from ethereum/execution-spec-tests#2256:

• Added test_mixed_sload_sstore with configurable SLOAD/SSTORE ratios (50-50, 70-30, 90-10)
• Added test_sload_empty_erc20_balanceof for pure SLOAD benchmarks
• Added test_sstore_erc20_approve for pure SSTORE benchmarks
• Support for parametrized contract counts (1, 5, 10, 20, 100)
• Uses pre-deployed ERC20 contracts via address stubs for execute mode

These benchmarks stress test client implementations with state-heavy operations across multiple contracts to identify performance bottlenecks.

✅ Checklist

• All: Ran fast tox checks to avoid unnecessary CI fails, see also Code Standards and Enabling Pre-commit Checks:

  uvx tox -e static

• All: PR title adheres to the repo standard - it will be used as the squash commit message and should start type(scope):.
• All: Considered adding an entry to CHANGELOG.md.
• All: Considered updating the online docs in the ./docs/ directory.
• All: Set appropriate labels for the changes (only maintainers can apply labels).
• Tests: Ran mkdocs serve locally and verified the auto-generated docs for new tests in the Test Case Reference are correctly formatted.
• Tests: For PRs implementing a missed test case, update the post-mortem document to add an entry the list.
• Ported Tests: All converted JSON/YML tests from ethereum/tests or tests/static have been assigned @ported_from marker.

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