Add callback interface class for serverside callback registration

[ChrisspyB]

Simple abstraction of the callback registration to allow registration of callbacks with the StoreHandler, rather than just an FDB object. Required for gribjump plugin on the databridge store servers, which at no point create an FDB object.

[ChrisspyB]

Related: ecmwf/gribjump#37. Since this change alters the callback interface slightly, gribjump tests will fail on the CI. This is expected.

[ChrisspyB]

Not ready for merge yet, some testing to do. But I am optimisitic.

[ChrisspyB]

Seems to work, ready to review

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 45.00000% with 11 lines in your changes missing coverage. Please review.

Project coverage is 61.21%. Comparing base (c6e8b9e) to head (2acd0cf).
Report is 222 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/fdb5/remote/server/StoreHandler.cc	0.00%	11 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #63      +/-   ##
===========================================
- Coverage    64.50%   61.21%   -3.30%     
===========================================
  Files          238      265      +27     
  Lines        13844    14935    +1091     
  Branches      1334     1474     +140     
===========================================
+ Hits          8930     9142     +212     
- Misses        4914     5793     +879     

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