[Config Service] Expose serverless contextRef (#156837)

docs/developer/architecture/core/configuration-service.asciidoc

@@ -147,3 +147,64 @@ export const config: PluginConfigDescriptor<ConfigType> = {
   ],
 };
 ----
+[[validating-your-configuration-based-on-context-references]]
+=== Validating your configuration based on context references
+Some features require special configuration when running in different modes (dev/prod/dist, or even serverless). For purpose, core injects the following _references_ in the validation's context:
+
+[cols="^1,^1,3"]
+|===
+|Context Reference |Potential values |Description
+
+|`dev`
+|`true`\|`false`
+|Is Kibana running in Dev mode?
+
+|`prod`
+|`true`\|`false`
+|Is Kibana running in Production mode (running from binary)?
+
+|`dist`
+|`true`\|`false`
+|Is Kibana running from a distributable build (not running from source)?
+
+|`serverless`
+|`true`\|`false`
+|Is Kibana running in Serverless offering?
+
+|`version`
+|`8.9.0`
+|The current version of Kibana
+
+|`buildNum`
+|`12345`
+|The build number
+
+|`branch`
+|`main`
+|The current branch running
+
+|`buildSha`
+|`12345`
+|The build SHA (typically refers to the last commit's SHA)
+
+|===
+
+To use any of the references listed above in a config validation schema, they can be accessed via `schema.contextRef('{CONTEXT_REFERENCE}')`:
+
+[source,js]
+----
+export const config = {
+  schema: schema.object({
+    // Enabled by default in Dev mode
+    enabled: schema.boolean({ defaultValue: schema.contextRef('dev') }),
+
+    // Setting only allowed in the Serverless offering
+    plansForWorldPeace: schema.conditional(
+      schema.contextRef('serverless'),
+      true,
+      schema.string({ defaultValue: 'Free hugs' }),
+      schema.never()
+    ),
+  }),
+};
+----