Add setting to mark paths varying by accept header as uncacheable

[MrKaplan-lw]

Requirements

• This is a feature request and not a bug report. Otherwise, please create a new bug report instead.
• Please check to see if this request (or a similar one) already exists.
• It's a single feature. Please don't request multiple features in one issue.

Describe the feature you'd like

As mentioned in #3099, various CDNs/caches don't support (as in ignore) the Vary header.
As a result, instance operators using a cache ignoring the vary header are required to do one of the following:

• disable caching entirely
• set up custom cache rules to disable caching for paths used by both Lemmy and Lemmy-UI
• set up custom cache keys if supported (Cloudflare locks this behind their enterprise plan)

To simplify setups, Lemmy-UI should have an option to explicitly mark paths used by both Lemmy and Lemmy-UI with cache-control: private, similar to how responses for authenticated users are treated.
This should not affect URLs that are exclusive to either Lemmy or Lemmy-UI, like static Lemmy-UI assets.

Related:

• [Bug]: Missing vary caching header #3099
• [Bug]: Incorrect vary caching header lemmy#5632
• Add setting to mark paths varying by accept header as uncacheable lemmy#5633