transform.cat should live in the cat namespace. (#54196)

* transform.cat should live in the cat namespace.

Similarly to to ml cat API's also living in the `cat` namespace.

Clients treat the `cat` namespace differently then other API's (return
types, content types). This introduces an exception to this rule.

* rename the specification file as well

x-pack/plugin/src/test/resources/rest-api-spec/api/transform.cat_transform.json → x-pack/plugin/src/test/resources/rest-api-spec/api/cat.transform.json

@@ -1,5 +1,5 @@
 {
-  "transform.cat_transform":{
+  "cat.transform":{
     "documentation":{
       "url":"https://www.elastic.co/guide/en/elasticsearch/reference/current/cat-transforms.html"
     },

x-pack/plugin/src/test/resources/rest-api-spec/test/transform/transforms_cat_apis.yml

@@ -51,7 +51,7 @@ teardown:
 ---
 "Test cat transform stats hiding headers":
   - do:
-      transform.cat_transform:
+      cat.transform:
         transform_id: "airline-transform-stats"
   - match:
       $body: |
@@ -61,7 +61,7 @@ teardown:
 ---
 "Test cat transform stats with column selection":
   - do:
-      transform.cat_transform:
+      cat.transform:
         transform_id: "airline-transform-stats"
         v: true
         h: id,version,source_index,dest_index,search_total,index_total,dt,cdtea,indexed_documents_exp_avg
@@ -90,7 +90,7 @@ teardown:
             "description": "description"
           }
   - do:
-      transform.cat_transform:
+      cat.transform:
         transform_id: "airline-transform-batch"
         v: true
   - match:
@@ -126,7 +126,7 @@ teardown:
             }
           }
   - do:
-      transform.cat_transform:
+      cat.transform:
         transform_id: "airline-transform-continuous"
         v: true
   - match: