Add Path Parameters to Cluster Allocation Explain API

The cluster allocation explain API now accepts parameters via the
request body, via path parameters passed in the URL, but not via both.

Issue: #127028

Author: joshua-adams-1

rest-api-spec/src/main/resources/rest-api-spec/api/cluster.allocation_explain.json

@@ -22,6 +22,22 @@
       ]
     },
     "params":{
+      "index": {
+        "type": "string",
+        "description": "Specifies the index within which we would like to describe the shard"
+      },
+      "shard": {
+        "type": "number",
+        "description": "Specifies the ID of the shard that you would like an explanation for"
+      },
+      "primary": {
+        "type":"boolean",
+        "description":"If true, returns explanation for the primary shard for the given shard ID"
+      },
+      "current_node": {
+        "type": "string",
+        "description": "Specifies the node ID or the name of the node to only explain a shard that is currently located on the specified node"
+      },
       "master_timeout":{
         "type":"time",
         "description":"Timeout for connection to master node"