[8.x] Add allow_partial_search_results and shard_failures to EQL (#3342) (#3358)

* Add spec for EQL allow_partial_search_results (#3342)

(cherry picked from commit 5987f79)

* Add shard_failures to the response

Author: luigidellaquila

output/openapi/elasticsearch-openapi.json

@@ -394,6 +394,8 @@
     "eql.search": {
       "request": [
         "Request: query parameter 'allow_no_indices' does not exist in the json spec",
+        "Request: query parameter 'allow_partial_search_results' does not exist in the json spec",
+        "Request: query parameter 'allow_partial_sequence_results' does not exist in the json spec",
         "Request: query parameter 'expand_wildcards' does not exist in the json spec",
         "Request: query parameter 'ignore_unavailable' does not exist in the json spec"
       ],