AK: Introduce UnixDateTime

This is a generic wrapper for a time instant relative to the unix epoch,
and does not account for leap seconds. It should be used in place of
Duration in most current cases.

Userland/Libraries/LibIPC/Decoder.cpp

@@ -76,6 +76,13 @@ ErrorOr<Duration> decode(Decoder& decoder)
     return AK::Duration::from_nanoseconds(nanoseconds);
 }
 
+template<>
+ErrorOr<UnixDateTime> decode(Decoder& decoder)
+{
+    auto nanoseconds = TRY(decoder.decode<i64>());
+    return AK::UnixDateTime::from_nanoseconds_since_epoch(nanoseconds);
+}
+
 template<>
 ErrorOr<URL> decode(Decoder& decoder)
 {