[ML] Add contextual ai endpoint API spec (elastic#135760)

Author: davidkyle

rest-api-spec/src/main/resources/rest-api-spec/api/inference.put_contextualai.json

@@ -0,0 +1,41 @@
+{
+  "inference.put_contextualai": {
+    "documentation": {
+      "url": "https://www.elastic.co/docs/api/doc/elasticsearch/operation/operation-inference-put-contextualai",
+      "description": "Create an Contextual AI inference endpoint"
+    },
+    "stability": "stable",
+    "visibility": "public",
+    "headers": {
+      "accept": [
+        "application/json"
+      ],
+      "content_type": [
+        "application/json"
+      ]
+    },
+    "url": {
+      "paths": [
+        {
+          "path": "/_inference/{task_type}/{contextualai_inference_id}",
+          "methods": [
+            "PUT"
+          ],
+          "parts": {
+            "task_type": {
+              "type": "string",
+              "description": "The task type"
+            },
+            "contextualai_inference_id": {
+              "type": "string",
+              "description": "The inference Id"
+            }
+          }
+        }
+      ]
+    },
+    "body": {
+      "description": "The inference endpoint's task and service settings"
+    }
+  }
+}