Enable resetting of ProxyPreferences

[koppor]

org.jabref.logic.net.ProxyPreferences needs to be rewritten.

See #14400 for the steps to do.

Furthermore, use the new "copy" logic to simplify the creation of backupProxyPreferences in org.jabref.gui.preferences.network.NetworkTabViewModel#NetworkTabViewModel.

[lalit2506verma]

/assign-me

[jabref-machine]

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