Add saving/loading for transforms, models, pipelines

[Mr-Geekman]

Before submitting (must do checklist)

• Did you read the contribution guide?
• Did you update the docs? We use Numpy format for all the methods and classes.
• Did you write any new necessary tests?
• Did you update the CHANGELOG?

Proposed Changes

• Add saving/loading for transforms, models, pipelines
• Add example for using saving/loading

Closing issues

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[codecov-commenter]

Codecov Report

Merging #1068 (b0a5987) into master (56990dd) will increase coverage by 0.46%.
The diff coverage is 99.00%.

@@            Coverage Diff             @@
##           master    #1068      +/-   ##
==========================================
+ Coverage   86.03%   86.49%   +0.46%     
==========================================
  Files         163      164       +1     
  Lines        8656     8901     +245     
==========================================
+ Hits         7447     7699     +252     
+ Misses       1209     1202       -7     

Impacted Files	Coverage Δ
etna/models/nn/mlp.py	100.00% <ø> (ø)
etna/core/saving.py	83.33% <83.33%> (ø)
etna/models/base.py	86.26% <88.88%> (-0.18%)	⬇️
etna/core/__init__.py	100.00% <100.00%> (ø)
etna/core/mixins.py	98.49% <100.00%> (+1.27%)	⬆️
etna/core/utils.py	97.56% <100.00%> (+1.40%)	⬆️
etna/ensembles/__init__.py	100.00% <100.00%> (ø)
etna/ensembles/direct_ensemble.py	94.64% <100.00%> (+0.09%)	⬆️
etna/ensembles/mixins.py	100.00% <100.00%> (ø)
etna/ensembles/stacking_ensemble.py	96.49% <100.00%> (+0.03%)	⬆️
... and 12 more

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