Add SaveMixin

[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

Look at #1002.

Closing issues

Closes #1002.

[codecov-commenter]

Codecov Report

Merging #1007 (d0b585f) into inference-v2 (8264118) will decrease coverage by 0.03%.
The diff coverage is 96.96%.

@@               Coverage Diff                @@
##           inference-v2    #1007      +/-   ##
================================================
- Coverage         85.79%   85.76%   -0.04%     
================================================
  Files               162      163       +1     
  Lines              8603     8666      +63     
================================================
+ Hits               7381     7432      +51     
- Misses             1222     1234      +12     

Impacted Files	Coverage Δ
etna/models/catboost.py	100.00% <ø> (ø)
etna/models/nn/mlp.py	100.00% <ø> (ø)
etna/models/nn/rnn.py	100.00% <ø> (ø)
etna/models/prophet.py	98.86% <ø> (ø)
etna/transforms/outliers/point_outliers.py	100.00% <ø> (ø)
etna/core/saving.py	83.33% <83.33%> (ø)
etna/core/__init__.py	100.00% <100.00%> (ø)
etna/core/mixins.py	98.30% <100.00%> (+1.08%)	⬆️
etna/models/base.py	86.59% <100.00%> (+0.15%)	⬆️
etna/transforms/base.py	96.22% <100.00%> (+0.07%)	⬆️
... and 2 more

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[github-actions]

🚀 Deployed on https://deploy-preview-1007--etna-docs.netlify.app

[martins0n]

Better to add key sort and indent param

[Mr-Geekman]

Fixed.

[martins0n]

👍