fix: index name parsing for datasources

To identify datasources in the index qualified names, they need to
be parsed into parts (separated only by dots). clusterQualifiedName
can't contain custom datasources, hence the distinction.

Signed-off-by: Sean Kao <seankao@amazon.com>

ppl/src/main/antlr/OpenSearchPPLParser.g4

@@ -309,7 +309,8 @@ tableSource
     ;
 
 clusterTableSource
-    : clusterQualifiedName
+    : qualifiedName
+    | clusterQualifiedName
     | ID_DATE_SUFFIX
     ;
 
@@ -600,7 +601,7 @@ qualifiedName
     ;
 
 clusterQualifiedName
-    : (ident COLON)? ident (DOT ident)*
+    : ident COLON ident (DOT ident)*
     ;
 
 wcQualifiedName