feat(pgs): forward etag and last-modified headers from imgproxy

[mac-chaffee]

This PR in addition to #178 complete the implementation of conditional requests (#177).

• Enabled last-modified headers from imgproxy: https://docs.imgproxy.net/configuration/options#IMGPROXY_USE_LAST_MODIFIED
• Refactored the content-type guessing and the routing logic for requests between minio and imgproxy into the ServeObject function
• ServeObject now returns an ObjectInfo struct so that we can inspect last-modified and etag headers coming from Minio/imgproxy

The result is that requests for images that flow through imgproxy will have the same etag and last-modified headers as files that we pull directly from minio. Example:

$ curl -iH "Host: picouser-test.pgs.dev.pico.sh" localhost:3000/avatar.jpg
HTTP/1.1 200 OK
Cache-Control: max-age=600
Cache-Status: Souin; fwd=uri-miss; stored; key=GET-http-picouser-test.pgs.dev.pico.sh-/avatar.jpg
Content-Length: 273114
Content-Type: image/jpeg
Etag: 915e552fd3a15fc0d5a8c5f2213efa32-1
Last-Modified: Sat, 21 Dec 2024 16:48:15 GMT
Surrogate-Key: picouser-test
Date: Sat, 21 Dec 2024 22:20:23 GMT

Warning: Binary output can mess up your terminal. Use "--output -" to tell 
Warning: curl to output it to your terminal anyway, or consider "--output 
Warning: <FILE>" to save to a file.


$ curl -iH "Host: picouser-test.pgs.dev.pico.sh" localhost:3000/avatar.jpg/ext:webp
HTTP/1.1 200 OK
Cache-Control: max-age=600
Cache-Status: Souin; fwd=uri-miss; stored; key=GET-http-picouser-test.pgs.dev.pico.sh-/avatar.jpg/ext:webp
Content-Length: 345164
Content-Type: image/webp
Etag: WHCbR8OqxLWx0vkUGegAbFZbcRkKZkkRFujaD2ewsS4/RIjkxNWU1NTJmZDNhMTVmYzBkNWE4YzVmMjIxM2VmYTMyLTEi
Last-Modified: Sat, 21 Dec 2024 21:48:14 GMT
Surrogate-Key: picouser-test
Date: Sat, 21 Dec 2024 22:20:26 GMT

Warning: Binary output can mess up your terminal. Use "--output -" to tell 
Warning: curl to output it to your terminal anyway, or consider "--output 
Warning: <FILE>" to save to a file.

(yes the etags in this example are missing quotes and the date is in the wrong timezone, two bugs which are fixed in #178)

[neurosnap]

nice!

[neurosnap]

Resolved the conflicts and merged, thanks!