[Framework] Add a DateTime property for CarrierJumpRequest DepartureTime (#119)

Refactored out the logic backing the JournalBase TimestampDateTime property so it can be used for any DateTime type property, providing a standardized json String -> DateTime conversion for any date-time property. Implemented as an `internal static` method on JournalBase so journal objects which inherit from JournalBase or don't inherit from it can use it.

Used this to provide a DepatureTimeDateTime on CarrierJumpRequest (this property was added in Update 14) and to implement the existing ExpiryDateTime on CurrentGoal.

From a quick search in the journal documentation, I don't see any other applications for this.

ObservatoryFramework/Files/Journal/FleetCarrier/CarrierJumpRequest.cs

@@ -1,4 +1,6 @@
-namespace Observatory.Framework.Files.Journal
+using System.Text.Json.Serialization;
+
+namespace Observatory.Framework.Files.Journal
 {
     public class CarrierJumpRequest : JournalBase
     {
@@ -9,5 +11,10 @@
         public string SystemName { get; init; }
         public ulong SystemID { get; init; }
         public string DepartureTime { get; init; }
+
+        [JsonIgnore]
+        public DateTime DepartureTimeDateTime {
+            get => ParseDateTime(DepartureTime);
+        }
     }
 }