feat: Request deduplication

[creativecreature]

Overview

This PR adds request deduplication by making the cache track in-flight requests for keys. This works for batchable endpoints too, as the cache is able to cherry-pick the keys it needs from different outgoing batches.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 84.05797% with 22 lines in your changes are missing coverage. Please review.

Project coverage is 89.43%. Comparing base (486dad1) to head (c384ac6).

Files	Patch %	Lines
passthrough.go	46.42%	9 Missing and 6 partials ⚠️
inflight.go	93.26%	4 Missing and 3 partials ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main       #8      +/-   ##
==========================================
- Coverage   90.29%   89.43%   -0.87%     
==========================================
  Files          10       11       +1     
  Lines         567      672     +105     
==========================================
+ Hits          512      601      +89     
- Misses         35       45      +10     
- Partials       20       26       +6     

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