feat(side-dag): add ability to update signers list

[glevco]

Depends on #1074

Motivation

Implement ability to update the signers list on PoA networks by configuring start and end block heights where each signer is active. By the default, a signer is active from height 0 to infinity. By setting the start height on a new signer, the network can coordinate to add it without disrupting existing blocks.

Acceptance Criteria

• Update PoA settings to be able to configure start_height and end_height for signers.
• Implement poa.get_active_signers() and update all usages of poa_settings.signers to use it instead.
• Update PoaBlockProducer to calculate the signer index depending on active signers.

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

Attention: Patch coverage is 94.33962% with 3 lines in your changes missing coverage. Please review.

Project coverage is 84.96%. Comparing base (d30a59c) to head (6bb8a4f).

Files	Patch %	Lines
hathor/consensus/poa/poa_block_producer.py	86.36%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1075      +/-   ##
==========================================
- Coverage   84.96%   84.96%   -0.01%     
==========================================
  Files         310      310              
  Lines       23492    23524      +32     
  Branches     3548     3554       +6     
==========================================
+ Hits        19961    19986      +25     
- Misses       2828     2835       +7     
  Partials      703      703              

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The base branch was changed.