RFR: 8343398: Add reducedData preference

[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.

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

Commit messages:
 - error message tweak
 - add macOS implementation
 - add linux implementation
 - Implementation of reducedData

Changes: https://git.openjdk.org/jfx/pull/1656/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1656&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343398
  Stats: 250 lines in 17 files changed: 211 ins; 28 del; 11 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