Raise EntityAlreadyExistsError for new endpoint handlers

[jfcalvo]

This PR includes some additional checks to new endpoints that are creating entities to check before hand if those entities exists.

These changes are included for the following endpoints:

• Create users.
• Create workspaces.
• Create workspace users.

[codecov]

Codecov Report

Patch coverage: 93.16% and project coverage change: +0.89 🎉

Comparison is base (8c05058) 92.74% compared to head (7f1a171) 93.63%.

Additional details and impacted files

@@                         Coverage Diff                         @@
##           feat/add-basic-database-support    #2556      +/-   ##
===================================================================
+ Coverage                            92.74%   93.63%   +0.89%     
===================================================================
  Files                                  167      163       -4     
  Lines                                 8271     8237      -34     
===================================================================
+ Hits                                  7671     7713      +42     
+ Misses                                 600      524      -76     

Flag	Coverage Δ
pytest	93.63% <93.16%> (+0.89%)	⬆️

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

Impacted Files	Coverage Δ
src/argilla/client/api.py	95.34% <ø> (ø)
src/argilla/client/sdk/text_classification/api.py	100.00% <ø> (+12.90%)	⬆️
...rc/argilla/server/daos/backend/mappings/helpers.py	97.22% <ø> (ø)
src/argilla/client/apis/datasets.py	92.62% <78.57%> (-4.58%)	⬇️
src/argilla/server/services/datasets.py	92.10% <84.21%> (-3.78%)	⬇️
src/argilla/client/client.py	93.07% <84.61%> (+6.25%)	⬆️
src/argilla/server/daos/backend/generic_elastic.py	86.18% <87.50%> (+0.39%)	⬆️
src/argilla/server/commons/telemetry.py	88.67% <88.88%> (+0.54%)	⬆️
.../argilla/server/apis/v0/handlers/records_search.py	93.87% <91.42%> (-6.13%)	⬇️
src/argilla/_constants.py	100.00% <100.00%> (ø)
... and 32 more

... and 2 files with indirect coverage changes

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