Feat/version2.0 endpoints

[lfbrehm]

This adds gRPC endpoint requests and missing database tests.

[codecov]

Codecov Report

Patch coverage: 8.69% and project coverage change: -0.82% ⚠️

Comparison is base (83a30b6) 21.23% compared to head (e1ec149) 20.42%.
Report is 1 commits behind head on feat/version2.0rework.

Additional details and impacted files

@@                    Coverage Diff                    @@
##           feat/version2.0rework      #99      +/-   ##
=========================================================
- Coverage                  21.23%   20.42%   -0.82%     
=========================================================
  Files                         43       51       +8     
  Lines                       3758     4549     +791     
=========================================================
+ Hits                         798      929     +131     
- Misses                      2960     3620     +660     

Files Changed	Coverage Δ
src/caching/cache.rs	37.50% <0.00%> (-4.11%)	⬇️
src/database/crud.rs	0.00% <ø> (ø)
src/database/dsls/pub_key_dsl.rs	87.50% <0.00%> (+87.50%)	⬆️
src/database/dsls/relation_type_dsl.rs	0.00% <0.00%> (ø)
src/grpc/endpoints.rs	0.00% <0.00%> (ø)
src/grpc/users.rs	0.00% <0.00%> (ø)
src/middlelayer/endpoints_db_handler.rs	0.00% <0.00%> (ø)
src/middlelayer/endpoints_request_types.rs	0.00% <0.00%> (ø)
src/middlelayer/snapshot_request_types.rs	0.00% <0.00%> (ø)
src/middlelayer/token_db_handler.rs	0.00% <0.00%> (ø)
... and 8 more

... and 1 file with indirect coverage changes

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[St4NNi]

LGTM, will do for now!