refactor(p2p): use Entrypoint type instead of str

[jansegre]

Motivation

Using a str is not ideal.

Acceptance Criteria

• introduce new Entrypoint class that can be parsed from and converted back to the tcp://hostname:port format, optionally allowing a peer-id (this was already supported): tcp://hostname:port/?id=peer-id
• introduce Hash class that works as a bytes with fixed length of 32, to be used as base for PeerId class that Entrypoint uses for storing peer-ids (this Hash class can be slowly used in more places in the future)
• replace every instance of str used as an endpoint description with Entrypoint

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 77.14286% with 40 lines in your changes missing coverage. Please review.

Project coverage is 85.06%. Comparing base (b3f3d3e) to head (c6cce1d).

Files	Patch %	Lines
hathor/p2p/manager.py	46.66%	13 Missing and 3 partials ⚠️
hathor/p2p/peer_discovery/dns.py	26.31%	14 Missing ⚠️
hathor/p2p/peer_id.py	62.50%	4 Missing and 2 partials ⚠️
hathor/p2p/resources/add_peers.py	76.92%	2 Missing and 1 partial ⚠️
hathor/p2p/entrypoint.py	98.30%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1086      +/-   ##
==========================================
- Coverage   85.08%   85.06%   -0.03%     
==========================================
  Files         309      310       +1     
  Lines       23396    23468      +72     
  Branches     3529     3544      +15     
==========================================
+ Hits        19906    19962      +56     
- Misses       2790     2806      +16     
  Partials      700      700              

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