move ninja objectives code into generic antag system (#20186)

Co-authored-by: deltanedas <@deltanedas:kde.org>

Content.Server/GenericAntag/GenericAntagComponent.cs

@@ -0,0 +1,30 @@
+using Content.Server.GameTicking.Rules.Components;
+using Robust.Shared.Prototypes;
+using Robust.Shared.Serialization.TypeSerializers.Implementations.Custom.Prototype;
+
+namespace Content.Server.GenericAntag;
+
+/// <summary>
+/// Added to a mob to make it a generic antagonist where all its objectives are fixed.
+/// This is unlike say traitor where it gets objectives picked randomly using difficulty.
+/// </summary>
+/// <remarks>
+/// A GenericAntag is not necessarily an antagonist, that depends on the roles you do or do not add after.
+/// </remarks>
+[RegisterComponent, Access(typeof(GenericAntagSystem))]
+public sealed partial class GenericAntagComponent : Component
+{
+    /// <summary>
+    /// Gamerule to start when a mind is added.
+    /// This must have <see cref="GenericAntagRuleComponent"/> or it will not work.
+    /// </summary>
+    [DataField(required: true), ViewVariables(VVAccess.ReadWrite)]
+    public EntProtoId Rule = string.Empty;
+
+    /// <summary>
+    /// The rule that's been spawned.
+    /// Used to prevent spawning multiple rules.
+    /// </summary>
+    [DataField, ViewVariables(VVAccess.ReadWrite)]
+    public EntityUid? RuleEntity;
+}