Add ListEncodedOption

[dknopik]

Add ListEncodedOption, which represents an Option via an ssz list with one or zero items:

/// An `Option<T>` that is SSZ-encoded as a `VariableList<T, 1>`.
///
/// `None` is encoded as an empty list and `Some(value)` as a single-element list.
/// This is useful for representing optional fields in SSZ containers without
/// relying on a union type.

This is e.g. used in partial data columns: ethereum/consensus-specs#4558

[codecov]

Codecov Report

❌ Patch coverage is 59.37500% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.21%. Comparing base (f3fcb7e) to head (b683bf5).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/list_encoded_option.rs	59.37%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #80      +/-   ##
==========================================
+ Coverage   50.10%   51.21%   +1.11%     
==========================================
  Files          13       14       +1     
  Lines         461      492      +31     
==========================================
+ Hits          231      252      +21     
- Misses        230      240      +10     

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[michaelsproul]

Looks good, thanks!