Handle multiple aliases in _cat/aliases api (#23698)

The alias parameter was documented as a list in our rest-spec, yet only the first value out of a list was getting read and processed. This commit adds support for multiple aliases to _cat/aliases

Closes #23661

core/src/main/java/org/elasticsearch/rest/action/cat/RestAliasAction.java

@@ -46,7 +46,7 @@ public class RestAliasAction extends AbstractCatAction {
     @Override
     protected RestChannelConsumer doCatRequest(final RestRequest request, final NodeClient client) {
         final GetAliasesRequest getAliasesRequest = request.hasParam("alias") ?
-                new GetAliasesRequest(request.param("alias")) :
+                new GetAliasesRequest(Strings.commaDelimitedListToStringArray(request.param("alias"))) :
                 new GetAliasesRequest();
         getAliasesRequest.local(request.paramAsBoolean("local", getAliasesRequest.local()));
 

docs/reference/cat/alias.asciidoc

@@ -54,5 +54,6 @@ alias4 test1 -      2            1,2
 The output shows that `alias2` has configured a filter, and specific routing
 configurations in `alias3` and `alias4`.
 
-If you only want to get information about a single alias, you can specify
-the alias in the URL, for example `/_cat/aliases/alias1`.
+If you only want to get information about specific aliases, you can specify 
+the aliases in comma-delimited format as a URL parameter, e.g., 
+/_cat/aliases/aliases/alias1,alias2.

rest-api-spec/src/main/resources/rest-api-spec/test/cat.aliases/10_basic.yaml

@@ -126,6 +126,52 @@
   - match:
       $body: / (^|\n)test_2 .+ \n/
 
+---
+"Multiple alias names":
+
+  - skip:
+      version: " - 5.99.99"
+      reason:  multiple aliases are supported only from 6.0.0 on
+
+  - do:
+        indices.create:
+            index: test
+
+  - do:
+        indices.create:
+            index: test2
+  - do:
+        indices.create:
+            index: test3
+
+  - do:
+        indices.put_alias:
+            index: test
+            name: foo
+
+  - do:
+        indices.put_alias:
+            index: test2
+            name: bar
+  - do:
+        indices.put_alias:
+            index: test3
+            name: baz
+
+  - do:
+        cat.aliases:
+            name: foo,bar
+            v: true
+            h: [alias, index]
+            s: [index]
+
+  - match:
+      $body: |
+               /^  alias \s+ index \n
+                   foo   \s+ test  \n
+                   bar   \s+ test2
+               $/
+
 ---
 "Column headers":