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 ssz_types so we don't have to continuously bump the version imported in context_deserialize

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.00%. Comparing base (2cc42ed) to head (09bba84).

Files with missing lines	Patch %	Lines
src/context_deserialize.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #73      +/-   ##
==========================================
- Coverage   59.39%   59.00%   -0.40%     
==========================================
  Files           9       10       +1     
  Lines         298      300       +2     
==========================================
  Hits          177      177              
- Misses        121      123       +2     

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