RFR: 8343398: Add reducedData preference [v4]

[Michael Strauß]

> The `reducedData` preference instructs applications to minimize internet traffic, as users might be on a metered network or a limited data plan.
> 
> This corresponds to the following OS settings:
> 
> Windows: Settings -> Network and Internet -> Ethernet/WiFi -> Metered connection
> macOS: Settings -> Network -> Ethernet/WiFi -> Network Settings -> Low data mode
> Ubuntu: Settings -> Network -> Wired/WiFi -> Metered connection
> 
> Change notifications work consistently on Windows and macOS. On my Ubuntu 24 system, the GIO `network-changed` signal is not sent when I only toggle the "metered connection" flag in network settings (and there's no signal specifically for low-data mode). The new value is only picked up when the connection changes by coming offline or going online.

Michael Strauß has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains seven additional commits since the last revision:

 - Merge branch 'master' into feature/reduced-data
 - refactor macOS PlatformSupport
 - javadoc change
 - error message tweak
 - add macOS implementation
 - add linux implementation
 - Implementation of reducedData

-------------

Changes:
  - all: https://git.openjdk.org/jfx/pull/1656/files
  - new: https://git.openjdk.org/jfx/pull/1656/files/8b8b1bdb..455613f0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1656&range=03
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1656&range=02-03

  Stats: 704 lines in 26 files changed: 292 ins; 356 del; 56 mod
  Patch: https://git.openjdk.org/jfx/pull/1656.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1656/head:pull/1656

PR: https://git.openjdk.org/jfx/pull/1656