feat: support callbacks for exists(), getState(), getExpireTime() methods

[AVaksman]

• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[codecov]

Codecov Report

Merging #1070 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1070   +/-   ##
=======================================
  Coverage   98.26%   98.26%           
=======================================
  Files          21       21           
  Lines       20423    20448   +25     
  Branches     1096     1096           
=======================================
+ Hits        20069    20094   +25     
  Misses        351      351           
  Partials        3        3           

Impacted Files	Coverage Δ
src/backup.ts	99.61% <100.00%> (+0.01%)	⬆️

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