Add new s3x package to replace storage

[rowanseymour]

We're moving away from storages which can be filesystem for testing, S3 for prod... and using S3 clients for everything with minio for local development and CI (and prod if anyone wants).

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 10 lines in your changes missing coverage. Please review.

Project coverage is 90.41%. Comparing base (6120347) to head (bf4c5d0).

Files	Patch %	Lines
s3x/s3.go	74.35%	5 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #131      +/-   ##
==========================================
- Coverage   90.86%   90.41%   -0.46%     
==========================================
  Files          45       47       +2     
  Lines        1707     1752      +45     
==========================================
+ Hits         1551     1584      +33     
- Misses        117      123       +6     
- Partials       39       45       +6     

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