Refine configsource interface

[pjanotti]

Small refinements to the configsource interface:

• Changing Retrieved to interface;
• Separating the watching capability in a separate interface: WatchableRetrieved;
• Adding a specific error to signal that the watched value was updated;

[codecov]

Codecov Report

Merging #2821 (358f128) into main (acb3ae1) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #2821   +/-   ##
=======================================
  Coverage   91.64%   91.64%           
=======================================
  Files         290      290           
  Lines       15541    15541           
=======================================
  Hits        14242    14242           
  Misses        889      889           
  Partials      410      410           

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