fixup: add timestamp and now helpers to LSPSDateTime

Add helper methods to LSPSDateTime that return the timestamp as seconds
and as milliseconds since the Unix epoch. These methods improve consistency
in date-time conversions.

Author: martinsaposnic

lightning-liquidity/src/lsps0/ser.rs

@@ -235,6 +235,17 @@ impl LSPSDateTime {
 			self.0.timestamp().try_into().expect("expiration to be ahead of unix epoch");
 		now_seconds_since_epoch > datetime_seconds_since_epoch
 	}
+
+	/// Returns the timestamp as seconds since the Unix epoch.
+	pub fn timestamp(&self) -> i64 {
+		self.0.timestamp()
+	}
+
+	/// Returns the timestamp as milliseconds since the Unix epoch.
+	pub fn now() -> Self {
+		Self(chrono::Utc::now())
+	}
+
 }
 
 impl FromStr for LSPSDateTime {