Proposal: Add Cache-Control header during file creation

[thangngoc89]

I'm serving large files directly from Cloudflare R2 by redirecting to pre-signed link for CDN benefits. See discussion.
However, with S3 or any S3 clones, you need to set Cache-Control header during file upload or modify afterwards. The current default is no caching.

Since the Serve function always add caching

pocketbase/tools/filesystem/filesystem.go

Line 395 in 5acaa0a

setHeaderIfMissing(res, "Cache-Control", "max-age=2592000, stale-while-revalidate=86400")

I would like to propose that Cache-Control should be added during writing to the file system.

I'm quite new to Golang but I've done some digging:

CacheControl is a field of WriterOptions and Pocketbase is currently calling this in 4 locations.

I could imagine a PR would involve adding this field to all of the 4 locations and fixing any failed test. If this is the case, I'm happy to send a PR.