Add data frame feature (#38934) (#39029)

The data frame plugin allows users to create feature indexes by pivoting a source index. In a
nutshell this can be understood as reindex supporting aggregations or similar to the so called entity
centric indexing.

Full history is provided in: feature/data-frame-transforms

docs/reference/rest-api/info.asciidoc

@@ -68,6 +68,11 @@ Example response:
         "available" : true,
         "enabled" : true
       },
+      "data_frame" : {
+          "description" : "Data Frame for the Elastic Stack",
+          "available" : true,
+          "enabled" : true
+      },
       "graph" : {
          "description" : "Graph Data Exploration for the Elastic Stack",
          "available" : true,