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 :warning:
...e/specializations/stores/web_service/webservice.py	62.50%	21 Missing :warning:
kgforge/specializations/stores/sparql_store.py	54.54%	5 Missing :warning:
kgforge/core/commons/execution.py	77.77%	2 Missing :warning:
kgforge/specializations/stores/nexus/service.py	50.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/BlueBrain/nexus-forge/pull/366/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BlueBrain) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/BlueBrain/nexus-forge/pull/366/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BlueBrain) | `74.49% <66.80%> (+0.12%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BlueBrain#carryforward-flags-in-the-pull-request-comment) to find out more.

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