Feat/version2.0 creation fixes

[lfbrehm]

• Adds FinishObjectStaging gRPC request
• Fixes unique constraint matching for create-requests
• Checks if user-specified endpoints exist for create-requests
• Adds tests for endpoints in tests/middlelayer/create.rs

[codecov]

Codecov Report

Patch coverage: 30.98% and project coverage change: +0.21% 🎉

Comparison is base (3955ba6) 37.73% compared to head (b5f9095) 37.95%.
Report is 79 commits behind head on feat/version2.0rework.

Additional details and impacted files

@@                    Coverage Diff                    @@
##           feat/version2.0rework     #110      +/-   ##
=========================================================
+ Coverage                  37.73%   37.95%   +0.21%     
=========================================================
  Files                         59       60       +1     
  Lines                       6741     7556     +815     
=========================================================
+ Hits                        2544     2868     +324     
- Misses                      4197     4688     +491     

Files Changed	Coverage Δ
src/auth/permission_handler.rs	0.00% <0.00%> (ø)
src/auth/structs.rs	7.27% <0.00%> (ø)
src/auth/token_handler.rs	0.00% <0.00%> (ø)
src/caching/notifications_handler.rs	0.00% <0.00%> (ø)
src/caching/structs.rs	27.58% <0.00%> (-45.15%)	⬇️
src/database/crud.rs	0.00% <ø> (ø)
src/database/dsls/external_user_id_dsl.rs	0.00% <0.00%> (ø)
src/database/dsls/identity_provider_dsl.rs	0.00% <0.00%> (ø)
src/database/dsls/mod.rs	0.00% <ø> (ø)
src/database/dsls/relation_type_dsl.rs	0.00% <0.00%> (ø)
... and 39 more

... and 2 files with indirect coverage changes

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[St4NNi]

LGTM