feat(side-dag): implement signer round-robin

[glevco]

Depends on #1071

Motivation

Improve the mechanism used in PoaBlockProducer to generate new block timestamps, as requested in #1061 (comment).

Acceptance Criteria

• Change poa.is_in_turn() to poa.in_turn_signer_index().
• Change PoaBlockProducer timestamp generation to use a signer round-robin approach instead of random delay.

Checklist

• If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.98%. Comparing base (eb74c11) to head (6c026f4).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1074      +/-   ##
==========================================
- Coverage   84.98%   84.98%   -0.01%     
==========================================
  Files         310      310              
  Lines       23495    23492       -3     
  Branches     3549     3548       -1     
==========================================
- Hits        19968    19964       -4     
- Misses       2824     2825       +1     
  Partials      703      703              

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The base branch was changed.