server: move mock relay into mock package

[MariusVanDerWijden]

📝 Summary

This PR moves the mock relay into its own package and moves some shared dependencies into their own packages in order to simplify the server package.

⛱ Motivation and Context

In order to simplify the architecture of mev-boost, it makes sense in my opinion to move certain functionalities into their own package. This makes it easier to reason about components and abstractions and its easier to add some more tests and refactors in the future.
This is just an initial PR to move the files, it changes no behavior. I will send some follow-up PRs to clean up the server code and the mock relay

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Codecov Report

Attention: Patch coverage is 67.01031% with 32 lines in your changes missing coverage. Please review.

Project coverage is 44.00%. Comparing base (8b662e0) to head (e555c15).
Report is 63 commits behind head on develop.

Files with missing lines	Patch %	Lines
server/mock/mock_relay.go	35.89%	25 Missing ⚠️
cli/types.go	0.00%	3 Missing ⚠️
server/service.go	89.47%	2 Missing ⚠️
server/types/U256str.go	0.00%	2 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #642      +/-   ##
===========================================
- Coverage    52.37%   44.00%   -8.38%     
===========================================
  Files           14       15       +1     
  Lines         1600     1609       +9     
===========================================
- Hits           838      708     -130     
- Misses         703      845     +142     
+ Partials        59       56       -3     

Flag	Coverage Δ
unittests	44.00% <67.01%> (-8.38%)	⬇️

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[jtraglia]

This looks reasonable to me. Thanks! One thing I'd like to be fixed first though.

[jtraglia]

LGTM 🥇

[metachris]

thanks!