feat: Add an API for getting all keys

[creativecreature]

Overview

This PR adds an API for getting all of the keys in the cache

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 91.11111% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 91.91%. Comparing base (3bc44ff) to head (70656b0).

Files	Patch %	Lines
cache.go	92.30%	1 Missing and 1 partial ⚠️
shard.go	77.77%	1 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #11      +/-   ##
==========================================
- Coverage   92.17%   91.91%   -0.27%     
==========================================
  Files          12       12              
  Lines         652      680      +28     
==========================================
+ Hits          601      625      +24     
- Misses         34       36       +2     
- Partials       17       19       +2     

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