Add optional context to register/update operations

[ssssarah]

Goal: provide schema context when registering updating/schema to then avoid having as_jsonld expand the context into the schemas, and have the schemas have a link to their context rather than the whole context
https://bbpteam.epfl.ch/project/issues/browse/DKE-1249

Still in progress, to be tested

[codecov-commenter]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (9ffefac) 74.13% compared to head (9611c02) 74.12%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #357      +/-   ##
==========================================
- Coverage   74.13%   74.12%   -0.01%     
==========================================
  Files          93       93              
  Lines        5919     5921       +2     
==========================================
+ Hits         4388     4389       +1     
- Misses       1531     1532       +1     

Flag	Coverage Δ
unittests	74.12% <72.22%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
kgforge/core/archetypes/store.py	59.36% <100.00%> (ø)
kgforge/specializations/stores/demo_store.py	39.41% <100.00%> (ø)
tests/specializations/stores/test_demo_store.py	100.00% <100.00%> (ø)
kgforge/core/forge.py	64.87% <66.66%> (+0.17%)	⬆️
kgforge/specializations/stores/bluebrain_nexus.py	28.57% <55.55%> (-0.07%)	⬇️

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