Add Backoff interface

[KengoWada]

Proposed changes

This PR refactors the Worker and Manager components to use the new Backoff interface introduced in issue #8. The goal is to make backoff policies more modular and customizable, allowing users to define their own retry behavior.

Related issue(s)

Fixes #8.

Checklist

Please ensure that the following checks have been performed:

• I have read and followed the project's contributing guidelines.
• My code follows the project's coding style.
• I have added or updated relevant documentation.
• I have added or updated tests (if applicable).
• All new and existing tests pass.
• My changes do not introduce any new warnings or errors.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 41.95%. Comparing base (e28424b) to head (1190330).

Files with missing lines	Patch %	Lines
manager.go	0.00%	2 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #12      +/-   ##
===========================================
+ Coverage    40.54%   41.95%   +1.41%     
===========================================
  Files            4        4              
  Lines          148      143       -5     
===========================================
  Hits            60       60              
+ Misses          88       83       -5     

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