Add context_deserialize impl

[macladson]

Moves the context_deserialize impl from context_deserialize to here. This will allow the implementation to automatically update to new versions of ethereum_ssz so we don't have to continuously bump the version we import in context_deserialize

[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.75%. Comparing base (d3741d3) to head (14e6311).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
ssz/src/context_deserialize.rs	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #61      +/-   ##
==========================================
- Coverage   76.13%   75.75%   -0.39%     
==========================================
  Files          13       14       +1     
  Lines         796      800       +4     
==========================================
  Hits          606      606              
- Misses        190      194       +4     

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