Add generic cache based on ttlcache

[rowanseymour]

and synced fetching logic used by mailroom asset loading

This should be usable for mailroom assets, courier channels and whatever we want to cache in temba-chat

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (424c323) 89.68% compared to head (e5af9ef) 90.25%.

Files	Patch %	Lines
cache/generic.go	93.87%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #112      +/-   ##
==========================================
+ Coverage   89.68%   90.25%   +0.56%     
==========================================
  Files          39       40       +1     
  Lines        1891     1940      +49     
==========================================
+ Hits         1696     1751      +55     
+ Misses        158      151       -7     
- Partials       37       38       +1     

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[rowanseymour]

this is something that mailroom asset loading doesn't do so I think under heavy load we're probably sometimes doing duplicate asset loads