Enhance observer handling for object store

[zussel]

Observer are still added per type but the methods use now variadic templates.

[codecov]

Codecov Report

Attention: Patch coverage is 92.24138% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 85.83%. Comparing base (286c50f) to head (7f94521).

Files	Patch %	Lines
include/matador/object/observer_list_creator.hpp	83.33%	3 Missing ⚠️
src/object/prototype_node.cpp	89.47%	2 Missing ⚠️
include/matador/object/object_store.hpp	95.83%	1 Missing ⚠️
...lude/matador/object/observer_list_copy_creator.hpp	93.75%	1 Missing ⚠️
include/matador/object/prototype_info.hpp	0.00%	1 Missing ⚠️
src/orm/session.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #164      +/-   ##
===========================================
+ Coverage    85.78%   85.83%   +0.05%     
===========================================
  Files          342      342              
  Lines        14468    14456      -12     
===========================================
- Hits         12412    12409       -3     
+ Misses        2056     2047       -9     

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