feat: Add optional referenceTaskIds for task followups (#608)

### Problem
Clients need to follow up or refine previously created tasks by agents.

### Proposal
**New Task in same contextId (Preferred)**

Context can be thought of as a larger goal from a client perspective.
Clients can use the same contextId from the previous task and send a
message for refinement or followup. Agents should spawn up new tasks.
Tasks cannot restart processing from terminal states (Completed, Failed
and Cancelled).

### Proposed API Change

* Addition of optional `referenceTaskIds` field in the `Message` object.
* Optional addition of `referenceArtifactId` in `part` metadata of
`Message` object.

Author: swapydapy

specification/json/a2a.json

@@ -1080,6 +1080,13 @@
                     },
                     "type": "array"
                 },
+                "referenceTaskIds": {
+                    "description": "list of tasks referenced as context by this message.",
+                    "items": {
+                        "type": "string"
+                    },
+                    "type": "array"
+                },
                 "role": {
                     "description": "Message sender's role",
                     "enum": [

types/src/types.ts

@@ -251,6 +251,8 @@ export interface Message {
   metadata?: {
     [key: string]: any;
   };
+  /** List of tasks referenced as context by this message.*/
+  referenceTaskIds?: string[];
   /** Identifier created by the message creator*/
   messageId: string;
   /** Identifier of task the message is related to */