Add WebServiceStore to query/search in external databases

[crisely09]

Added web service store as part of the effort to reach external databases, using the new structure of forge stores.

[codecov-commenter]

Codecov Report

Attention: 82 lines in your changes are missing coverage. Please review.

Comparison is base (2e616cd) 74.36% compared to head (47e8464) 74.49%.

Files	Patch %	Lines
...gforge/specializations/stores/web_service_store.py	50.46%	53 Missing ⚠️
...e/specializations/stores/web_service/webservice.py	62.50%	21 Missing ⚠️
kgforge/specializations/stores/sparql_store.py	54.54%	5 Missing ⚠️
kgforge/core/commons/execution.py	77.77%	2 Missing ⚠️
kgforge/specializations/stores/nexus/service.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #366      +/-   ##
==========================================
+ Coverage   74.36%   74.49%   +0.12%     
==========================================
  Files         100      103       +3     
  Lines        6315     6531     +216     
==========================================
+ Hits         4696     4865     +169     
- Misses       1619     1666      +47     

Flag	Coverage Δ
unittests	74.49% <66.80%> (+0.12%)	⬆️

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