Backend: implement Silent Payments sending #3875

Job	Run time
linux-github--dotnet-and-newmono	7m 24s
linux-vanilla--newmono-only	9m 58s
linux-github--dotnet-and-mono	4m 21s
linux-vanilla--stockdotnet6-and-newmono	6m 7s
linux-vanilla--stockdotnet6-and-stockmono	6m 58s
linux-vanilla--stockdotnet6-only	5m 32s
linux-vanilla--stockmono-only	4m 20s
macOS--dotnet6-and-mono	11m 53s
macOS--dotnet6-only	4m 43s
macOS--mono-only	11m 24s
windows--dotnet6-and-legacyFramework	6m 34s
windows--dotnet6-only	6m 37s
windows--legacyFramework-only	12m 45s
conventions	0s
snap_pkg_beta	0s
snap_pkg	0s
	1h 38m 36s

Provide feedback