Is there any "local"/minio-esque object store properly implements the full conditional GET/PUT/DELETE spec, ETag, etc.? I have struggled to find this in the past.

I implemented this (HS5). Etags match the AWS S3 ones as well.

Awesome, I will take a look! We need something for our end-to-end tests at work that we can use and trivially spin up buckets for millions of tests in parallel.