Add RuntimeVariableList and RuntimeFixedVector

[macladson]

Moves the RuntimeVariableList and RuntimeFixedVector types from Lighthouse to ssz_types which is a more natural place for them to exist.

They have some additional dependencies so I have gated them behind the runtime-types feature.

[codecov]

Codecov Report

❌ Patch coverage is 27.58621% with 84 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.36%. Comparing base (2d4eb64) to head (d825a64).

Files with missing lines	Patch %	Lines
src/runtime_types/runtime_variable_list.rs	35.55%	58 Missing ⚠️
src/runtime_types/runtime_fixed_vector.rs	0.00%	26 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #67      +/-   ##
==========================================
- Coverage   59.25%   50.36%   -8.90%     
==========================================
  Files           8       10       +2     
  Lines         297      413     +116     
==========================================
+ Hits          176      208      +32     
- Misses        121      205      +84     

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