[nm-1.42] [RFE] dispatch `dns-change` dispatcher event
Backport the dns-change
event feature to nm-1.42 branch.
The git cherry-pick -x d10f20fd01a7bb3225c7e38ed80449e19156344b 3cdce71b95cea11bf409d9353c35a4dea6f33984
works well without conflicts.
Manually tested on dns-change event.