Introduce stability description to the REST API specification (#38413)

* introduce state to the REST API specification

* change state over to stability

* CCR is no GA updated to stable

* SQL is now GA so marked as stable

* Introduce `internal` as state for API's, marks stable in terms of lifetime but unstable in terms of guarantees on its output format since it exposes internal representations

* make setting a wrong stability value, or not setting it at all an error that causes the YAML test suite to fail

* update spec files to be explicit about their stability state

* Document the fact that stability needs to be defined

Otherwise the YAML test runner will fail (with a nice exception message)

* address check style violations

* update rest spec unit tests to include stability

* found one more test spec file not declaring stability, made sure stability appears after documentation everywhere

* cluster.state is stable, mark response in some way to denote its a key value format that can be changed during minors

* mark data frame API's as beta

* remove internal and private as states for an API

* removed the wrong enum values in the Stability Enum in the previous commit

Author: Mpdreamz

modules/lang-painless/src/test/resources/rest-api-spec/api/scripts_painless_context.json

@@ -1,5 +1,6 @@
 {
   "scripts_painless_context": {
+    "stability": "experimental",
     "methods": ["GET"],
     "url": {
       "paths": ["/_scripts/painless/_context"],

plugins/examples/rest-handler/src/test/resources/rest-api-spec/api/cat.example.json

@@ -1,6 +1,7 @@
 {
   "cat.example": {
     "documentation": "",
+    "stability" : "stable",
     "methods": ["GET"],
     "url": {
       "paths": ["/_cat/example"],