Support ETags for /ocs/v2.php/cloud/users/{userId} endpoint (or all ocs endpoints?)

[stefan-niedermann]

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Is your feature request related to a problem? Please describe.
Motivation is to keep the transmitted data traffic from clients to the server as low as possible for 1. a better performance and 2. to preserve mobile traffic

Describe the solution you'd like
Support ETags in all ocs endpoints by default, but i will already be happy if this endpoint supports it:
/ocs/v2.php/cloud/users/{userId}

Describe alternatives you've considered
Not implementing human readable display names but technical user IDs because it's not worth the traffic without ETags in the Notes app.

Additional context
Some time ago someone already implemented this for the capabilities endpoint, maybe it's just a bit copy & paste...

cc @nickvergessen who pointed me here: https://github.com/nextcloud/notifications/blob/5ac74461141d8992bf1fb22c6e5fabf0544e5db1/lib/Controller/EndpointController.php#L128

[kesselb]

Some time ago someone already implemented this for the capabilities endpoint,

#20722

maybe it's just a bit copy & paste...

Yeah. We should introduce a proper way to do it. Otherwise this kind of change is done in every controller. We talked about a middleware to automatically calculate the etag for each response. But we still need some allow-/blocklist for exclude responses.