Add a flag to the reindex command to force a full reindex

That can be helpful when the search service configuration has changed,
e.g. by enabling TIKA. Previously files that had already been indexed
were not indexed again and thus were no part of the fulltext index.

Fixes #2285
Fixes #2578

protogen/proto/opencloud/services/search/v0/search.proto

@@ -104,6 +104,7 @@ message SearchIndexResponse {
 message IndexSpaceRequest {
   string space_id = 1;
   string user_id = 2;
+  bool force_reindex = 3;
 }
 
 message IndexSpaceResponse {