feat: add parent_id column to workspace_agents table (#17758)

Adds a new nullable column `parent_id` to `workspace_agents` table. This
lays the groundwork for having child agents.

coderd/apidoc/docs.go

@@ -17021,6 +17021,14 @@ const docTemplate = `{
                 "operating_system": {
                     "type": "string"
                 },
+                "parent_id": {
+                    "format": "uuid",
+                    "allOf": [
+                        {
+                            "$ref": "#/definitions/uuid.NullUUID"
+                        }
+                    ]
+                },
                 "ready_at": {
                     "type": "string",
                     "format": "date-time"
@@ -19033,6 +19041,18 @@ const docTemplate = `{
         "url.Userinfo": {
             "type": "object"
         },
+        "uuid.NullUUID": {
+            "type": "object",
+            "properties": {
+                "uuid": {
+                    "type": "string"
+                },
+                "valid": {
+                    "description": "Valid is true if UUID is not NULL",
+                    "type": "boolean"
+                }
+            }
+        },
         "workspaceapps.AccessMethod": {
             "type": "string",
             "enum": [