extract serializer and allow passing extra descriptors

[gimmyxd]

Allow adding extra descriptors when creating a directory client. The descriptors are added to the registry that is used for message serialization to JSON.
By default the directory client has a registry that knows about all the types in pb-directory. This change allows extending the directory client with the types defined in a separate namespace(eg: pb-directory-store).

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.07%. Comparing base (3066505) to head (fa55bef).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #111      +/-   ##
==========================================
+ Coverage   88.03%   88.07%   +0.03%     
==========================================
  Files          31       31              
  Lines         903      906       +3     
  Branches      257      262       +5     
==========================================
+ Hits          795      798       +3     
+ Misses        108      107       -1     
- Partials        0        1       +1     

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