Backend: implement Silent Payments sending #3890

Job	Run time
windows--dotnet6-and-legacyFramework	5m 20s
macOS--mono-only	11m 13s
linux-github--dotnet-and-mono	3m 42s
linux-github--dotnet-and-newmono	7m 24s
linux-vanilla--stockdotnet6-only	5m 4s
linux-vanilla--stockdotnet6-and-newmono	5m 48s
linux-vanilla--stockmono-only	5m 22s
macOS--dotnet6-and-mono	11m 42s
macOS--dotnet6-only	4m 24s
windows--dotnet6-only	7m 7s
windows--legacyFramework-only	15m 37s
linux-vanilla--newmono-only	8m 30s
linux-vanilla--stockdotnet6-and-stockmono	6m 40s
conventions	1m 15s
snap_pkg_beta	6m 37s
snap_pkg	5m 52s
	1h 51m 37s

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