Add sparse_encoder to models.encodings

[renatomello]

This is based on Section IV of https://arxiv.org/abs/2405.20408

Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.

[codecov]

Codecov Report

Attention: Patch coverage is 6.45161% with 58 lines in your changes missing coverage. Please review.

Project coverage is 99.07%. Comparing base (5c93898) to head (c9f76f9).

Files with missing lines	Patch %	Lines
src/qibo/models/encodings.py	3.33%	58 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1591      +/-   ##
==========================================
- Coverage   99.58%   99.07%   -0.51%     
==========================================
  Files          76       76              
  Lines       11322    11382      +60     
==========================================
+ Hits        11275    11277       +2     
- Misses         47      105      +58     

Flag	Coverage Δ
unittests	99.07% <6.45%> (-0.51%)	⬇️

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