Add noop block simulator

[ferranbt]

📝 Summary

This PR adds an alternative implementation for the block simulator which always returns nil. This is useful in testing scenarios where you want to run without a block simulator as we do in https://github.com/flashbots/builder-playground.

⛱ Motivation and Context

📚 References

---

✅ I have run these commands

• make lint
• make test-race
• go mod tidy
• I have seen and agree to CONTRIBUTING.md

[codecov-commenter]

Codecov Report

❌ Patch coverage is 64.28571% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 29.51%. Comparing base (c71fc35) to head (8ab682e).
⚠️ Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
services/api/service.go	62.50%	1 Missing and 2 partials ⚠️
services/api/blocksim_ratelimiter.go	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #772      +/-   ##
==========================================
- Coverage   30.47%   29.51%   -0.97%     
==========================================
  Files          42       42              
  Lines        7236     7790     +554     
==========================================
+ Hits         2205     2299      +94     
- Misses       4752     5207     +455     
- Partials      279      284       +5     

Flag	Coverage Δ
unittests	29.51% <64.28%> (-0.97%)	⬇️

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