Add extension

[maypok86]

Description

Added Extension feature.

Related issue(s)

• Resolves Request Get Expiration With Get #56

✅ Checklist for your Pull Request

Related changes (conditional)

• Tests

  • If I added new functionality, I added tests covering it.
  • If I fixed a bug, I added a regression test to prevent the bug from
    silently reappearing again.

• Documentation

  • I checked whether I should update the docs and did so if necessary:
    • README

• Public contracts

  • My changes doesn't break project license.

Stylistic guide (mandatory)

• My code complies with the styles guide.

• My commit history is clean (only contains changes relating to my
  issue/pull request and no reverted-my-earlier-commit changes) and commit
  messages start with identifiers of related issues in square brackets.

  Example: [#42] Short commit description

  If necessary both of these can be achieved even after the commits have been
  made/pushed using rebase and squash.

Before merging (mandatory)

• Check target branch of PR is set correctly

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.87755% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 95.87%. Comparing base (18da5c2) to head (f7746d8).

Files	Patch %	Lines
internal/core/cache.go	86.84%	3 Missing and 2 partials ⚠️
internal/s3fifo/small.go	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #65      +/-   ##
==========================================
+ Coverage   95.43%   95.87%   +0.44%     
==========================================
  Files          24       26       +2     
  Lines        1314     1382      +68     
==========================================
+ Hits         1254     1325      +71     
+ Misses         37       35       -2     
+ Partials       23       22       -1     

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