When S3 storage is turned on, ETag is not returned

[lognaturel]

Problem description

When a server is configured to store blobs in S3, responses to OpenRosa requests (and likely other requests) for those blobs don't include ETag headers. This means that browser caching can't be taken advantage of.

URL of the page

https://test.getodk.cloud/v1/key/uQpSMt9QOnZHtq1We9Zveqo3VR0u7!L5UXIdvXbui1n1KkFPEaUA$fmYfWYzEsRA/projects/523/forms/geojsonUpdateBug/attachments/abc.geojson

Steps to reproduce the problem

• Enable S3 storage on a server
• Upload a file
• Request that file through the OpenRosa API

Expected behavior

Central uses information it already has like the file's md5 hash as the ETag so that it can return a 304 if the client already has the file cached.

Central version shown in version.txt

versions:
801ab503c74eaa24b27a226622eceee9351d5db0 (v2024.2.1-10-g801ab50)
+5e95312bc6177ed79f170d8851da39fe327219d4 client (v2024.2.1-63-g5e95312b)
+4355d9e4c3a0f7bb6b694aad4cf318145905da2e server (v2024.2.0-114-g4355d9e4)

Browser version

Firefox 132.0.2 (aarch64)

Notes

Discovered while looking into #662