Add the REST API specifications for SLM Status / Start / Stop endpoints. (#49759)

Was originally missed in PR #47710

(cherry picked from commit 133b34c8355639ae0f699a86ffd9f37d19f73bca)

x-pack/plugin/src/test/resources/rest-api-spec/api/slm.get_status.json

@@ -0,0 +1,19 @@
+{
+  "slm.get_status":{
+    "documentation":{
+      "url":"https://www.elastic.co/guide/en/elasticsearch/reference/current/slm-get-status.html"
+    },
+    "stability":"stable",
+    "url":{
+      "paths":[
+        {
+          "path":"/_slm/status",
+          "methods":[
+            "GET"
+          ]
+        }
+      ]
+    },
+    "params":{}
+  }
+}

x-pack/plugin/src/test/resources/rest-api-spec/api/slm.start.json

@@ -0,0 +1,19 @@
+{
+  "slm.start":{
+    "documentation":{
+      "url":"https://www.elastic.co/guide/en/elasticsearch/reference/current/slm-start.html"
+    },
+    "stability":"stable",
+    "url":{
+      "paths":[
+        {
+          "path":"/_slm/start",
+          "methods":[
+            "POST"
+          ]
+        }
+      ]
+    },
+    "params":{}
+  }
+}

x-pack/plugin/src/test/resources/rest-api-spec/api/slm.stop.json

@@ -0,0 +1,19 @@
+{
+  "slm.stop":{
+    "documentation":{
+      "url":"https://www.elastic.co/guide/en/elasticsearch/reference/current/slm-stop.html"
+    },
+    "stability":"stable",
+    "url":{
+      "paths":[
+        {
+          "path":"/_slm/stop",
+          "methods":[
+            "POST"
+          ]
+        }
+      ]
+    },
+    "params":{}
+  }
+}