[DomDistiller] Add text_content field to DomDistillerResult

Changes to Distribution repository.

Add optional string text_content to DomDistillerResult proto. The new
field allows extracting clean text content for downstream TTS consumers.

Bug: 552967247
Change-Id: I084de64d9a461f723485cff2721873213039d0a6

proto/dom_distiller.proto

diff --git a/proto/dom_distiller.proto b/proto/dom_distiller.proto
index a8a5aaf..afa99c5 100644
--- a/proto/dom_distiller.proto
+++ b/proto/dom_distiller.proto
@@ -89,6 +89,7 @@
   }
 
   repeated ContentImage content_images = 10;
+  optional string text_content = 11;
 }
 
 message DomDistillerOptions {