feat: introducing BlockComponent

[ksn6]

Problem and Summary of Changes

A BlockComponent represents either a batch of entries or a special block marker. Most of the time, a block component contains a vector of entries. However, periodically, there are special messages that a block needs to contain. To accommodate these special messages, BlockComponent allows for the inclusion of special data via VersionedBlockMarker.

Currently supported special entry types include:

• BlockFooter: Contains metadata about block production
• UpdateParent: Used in optimistic block packing algorithms for Alpenglow

Additional special entry types may be added in the future.

This data structure addresses two SIMDs:

• SIMD-0307: Add Block Footer solana-foundation/solana-improvement-documents#307
• SIMD-0337: Markers for Alpenglow Fast Leader Handover solana-foundation/solana-improvement-documents#337

[codecov-commenter]

Codecov Report

❌ Patch coverage is 93.48404% with 98 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.0%. Comparing base (5453ef6) to head (ffc912d).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master    #8100     +/-   ##
=========================================
  Coverage    82.9%    83.0%             
=========================================
  Files         823      824      +1     
  Lines      360873   362377   +1504     
=========================================
+ Hits       299346   300792   +1446     
- Misses      61527    61585     +58     

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[ksn6]

block_component.rs is changing pretty rapidly over in alpenglow. Once that's stabilized, we'll upstream this change.