perf: cache pre-fetched mailboxes on the HTTP level

[st3iny]

Fix #11282

Each mailbox gets assigned with an etag which is generated from its id and all three sync tokens. This etag will be included in pre-fetch requests and the server will enable caching for the response.

If a mailbox is changed in the meantime, the etag will be different, resulting in a different URL which will cause the client to refetch the cached first page.

Etags are injected via initial state.

The cache duration of a week (immutable) is up for discussion. Please keep in mind that the newest state will always be fetched once a user opens a mailbox. The pre-fetch mechanism is more about preventing a loading skeleton when first opening a mailbox.

[ChristophWurst]

Looks good

[ChristophWurst]

If I read the Firefox network logs correctly the requests are not cached. I have turned on HTTP caching. Other requests, mostly images, are cached.

cache-control: private, max-age=604800, immutable

^ header is sent, but doesn't have the expected effect

[ChristophWurst]

It works with Chromium. It says (disk cache).

[ChristophWurst]

Spent more time than I should and still can't figure out why Firefox is not caching XHRs despite the cache headers

[ChristophWurst]

Works in Chromium, Firefox situation unclear, could be my setup

[kesselb]

Nice, Thank you!

I expected something different when reading "ETag". It looks more like a cache buster, and the server will not respond with 204 status.

[st3iny]

I renamed the parameter from etag to v to be more consistent with the rest of Nextcloud. I effectively implemented a cache buster rather than actually providing ETag functionality.

(The member of the serialized mailbox is called cacheBuster.)

Thanks @kesselb for the suggestion.

[st3iny]

I included some unit tests ...

[st3iny]

/backport to stable5.1