Made Run, Model, ModelVersion and Project made first class citizens

[Raalsky]

This allows us to call Run and any other object in-place of init_run:

with Run() as run:
    run['a'] = 99.9

---

Before submitting checklist

• Did you update the CHANGELOG? (not for test updates, internal changes/refactors or CI/CD setup)
• Did you asked the docs owner to review all the updated user-facing interfaces?

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -71.52 ⚠️

Comparison is base (c547756) 71.51% compared to head (7f8a3ad) 0.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #1246       +/-   ##
==========================================
- Coverage   71.51%   0.00%   -71.52%     
==========================================
  Files         282     277        -5     
  Lines       13643   13583       -60     
==========================================
- Hits         9757       0     -9757     
- Misses       3886   13583     +9697     

Flag	Coverage Δ
e2e	?
unit	?
unit-pull-request	0.00% <0.00%> (?)

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

Impacted Files	Coverage Δ
src/neptune/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
.../neptune/internal/backends/neptune_backend_mock.py	0.00% <ø> (-38.39%)	⬇️
.../neptune/metadata_containers/metadata_container.py	0.00% <0.00%> (-87.31%)	⬇️
src/neptune/metadata_containers/model.py	0.00% <0.00%> (-94.12%)	⬇️
src/neptune/metadata_containers/model_version.py	0.00% <0.00%> (-91.67%)	⬇️
src/neptune/metadata_containers/project.py	0.00% <0.00%> (-90.48%)	⬇️
src/neptune/metadata_containers/run.py	0.00% <0.00%> (-87.18%)	⬇️
src/neptune/envs.py	0.00% <0.00%> (-100.00%)	⬇️
src/neptune/constants.py	0.00% <0.00%> (-100.00%)	⬇️
src/neptune/common/envs.py	0.00% <0.00%> (-100.00%)	⬇️
... and 237 more

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[twolodzko]

I didn't review the implementation, just looked at the API, and see no reason why we shouldn't have it.

[AleksanderWWW]

Looks good to me on the code front. Doc strings probably need to be updated, as they only point to using init_* aliases

[Raalsky]

@normandy7, this is mostly our internal code deduplication etc. but as a result, this adds a feature to use Run/Model/ModelVersion/Project in addition to standard init_run. There is no requirement to expose it currently in docs/docstrings. For example:

with Run() as run:
    run['a'] = 99.9

# or

run = Run()

[normandy7]

I agree no docs action should be required for now. We may update the class docstrings in the future, should we decide to "officially" include the constructor approach in the API. Thanks!