feature: implement proposer interface for consensus

[rianhughes]

This PR implements the Proposer interface for the consensus layer. The proposer is responsible for generating block proposals, handling both empty and non-empty blocks depending on whether the mempool receives transactions within a given time window.

Notes:
Proposal timing: The caller is responsible for deciding:
* When to stop waiting for transactions and proceed with an empty block.
* When to stop executing transactions (in the non-empty block flow) and continue with proposal finalisation.

Gas strategy: This implementation reuses gas values from the previous block. Dynamic gas fee computation is out of scope of this PR, and probably requires SNs input.

Code reuse: Some logic is shared with the Validator PR. Either PR can be merged first without conflict.

closes #2887

[codecov]

Codecov Report

Attention: Patch coverage is 82.67717% with 22 lines in your changes missing coverage. Please review.

Project coverage is 74.49%. Comparing base (97d4c84) to head (e2d1ee3).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
consensus/proposer/proposer.go	80.55%	15 Missing and 6 partials ⚠️
builder/builder.go	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2873      +/-   ##
==========================================
+ Coverage   74.40%   74.49%   +0.08%     
==========================================
  Files         229      230       +1     
  Lines       24867    24983     +116     
==========================================
+ Hits        18503    18610     +107     
- Misses       5134     5140       +6     
- Partials     1230     1233       +3     

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[rodrigo-pino]

Looks good overall, will give a deeper check later. I appreciate the mindfulness of how input and output parameters are used by reference/value. Thank you!