Implement EIP-7916: ProgressiveBitList

[macladson]

Proposed Changes

Implements the relevant parts of EIP-7916 (ProgressiveBitList).

This is done by adding Progressive as an available BitfieldBehaviour option.
Encoding/decoding behaviour is identical to Bitfield<Variable<N>> so there is some code duplciation. We could consider defining a shared trait for both variable length behaviours and deduplicate this.

Progressive merkleization will be implemented in tree_hash crate, this PR just defines these types which we become implementation targets later

Reference

https://eips.ethereum.org/EIPS/eip-7916

[codecov]

Codecov Report

❌ Patch coverage is 78.33333% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.74%. Comparing base (bc82946) to head (6a686f4).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
ssz/src/bitfield/progressive.rs	78.33%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #63      +/-   ##
==========================================
+ Coverage   76.62%   76.74%   +0.11%     
==========================================
  Files          14       15       +1     
  Lines         800      860      +60     
==========================================
+ Hits          613      660      +47     
- Misses        187      200      +13     

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