feat(side-dag): implement PoaBlockProducer

[glevco]

Depends on #1060

Motivation

Finish the Proof-of-Authority side-dag implementation by implementing the PoaBlockProducer which is analogous to a mining service, in the full node itself.

Acceptance Criteria

• Implement poa module and PoaBlockProducer.
• Implement gen_genesis CLI command.

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 47.51773% with 74 lines in your changes missing coverage. Please review.

Project coverage is 84.62%. Comparing base (7c7c261) to head (b85b855).

Files	Patch %	Lines
hathor/consensus/poa/poa_block_producer.py	27.05%	62 Missing ⚠️
hathor/builder/cli_builder.py	14.28%	5 Missing and 1 partial ⚠️
hathor/manager.py	33.33%	2 Missing and 2 partials ⚠️
hathor/consensus/poa/poa_signer.py	83.33%	1 Missing ⚠️
hathor/mining/block_template.py	91.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1061      +/-   ##
==========================================
- Coverage   84.92%   84.62%   -0.30%     
==========================================
  Files         307      308       +1     
  Lines       23177    23295     +118     
  Branches     3503     3518      +15     
==========================================
+ Hits        19683    19714      +31     
- Misses       2792     2870      +78     
- Partials      702      711       +9     

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The base branch was changed.