feat(consensus): add ProposalParts and related objects

[guy-starkware]

This PR adds ProposalParts which is an enum that can be ProposalInit, Transactions, or ProposalFin.
This will be the content that we stream whenever Consensus wants to make a new proposal.

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This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 86.25592% with 29 lines in your changes missing coverage. Please review.

Project coverage is 74.42%. Comparing base (693d1df) to head (dfa7bfc).
Report is 70 commits behind head on main.

Files with missing lines	Patch %	Lines
...rates/papyrus_protobuf/src/converters/consensus.rs	76.19%	9 Missing and 6 partials ⚠️
...sequencing/papyrus_consensus/src/stream_handler.rs	90.00%	13 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #898      +/-   ##
==========================================
- Coverage   76.04%   74.42%   -1.62%     
==========================================
  Files         359      358       -1     
  Lines       38298    36434    -1864     
  Branches    38298    36434    -1864     
==========================================
- Hits        29122    27117    -2005     
- Misses       6841     7173     +332     
+ Partials     2335     2144     -191     

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