Add project_routing param to _project/tags

Author: smalyshev

specification/_json_spec/project.tags.json

@@ -7,15 +7,26 @@
     "stability": "stable",
     "visibility": "public",
     "headers": {
-      "accept": ["application/json"]
+      "accept": [
+        "application/json"
+      ]
     },
     "url": {
       "paths": [
         {
           "path": "/_project/tags",
-          "methods": ["GET"]
+          "methods": [
+            "GET",
+            "POST"
+          ]
         }
       ]
+    },
+    "params": {
+      "project_routing": {
+        "type": "string",
+        "description": "A Lucene query using project metadata tags to limit which projects to search, such as _alias:_origin or _alias:*pr*. Only supported in serverless."
+      }
     }
   }
 }