feat!: use base instead of interface for ProtocolClient class

[JKRhb]

As a step towards adding #167, this PR changes the class modifier of the ProtocolClient class from interface to class, making it possible to specify certain (discovery) functionalities via mixins.

As they alter the API of the core library, these are breaking changes.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.74%. Comparing base (9e1ac1f) to head (26a8075).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #168      +/-   ##
==========================================
+ Coverage   66.69%   66.74%   +0.05%     
==========================================
  Files          77       77              
  Lines        2438     2436       -2     
==========================================
  Hits         1626     1626              
+ Misses        812      810       -2     

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