Add the fts5 'optimize' command.

FossilOrigin-Name: e749be563d8e738af113bd301770e2f22763ab77

ext/fts5/fts5_storage.c

@@ -585,6 +585,10 @@ int sqlite3Fts5StorageRebuild(Fts5Storage *p){
   return rc;
 }
 
+int sqlite3Fts5StorageOptimize(Fts5Storage *p){
+  return sqlite3Fts5IndexOptimize(p->pIndex);
+}
+
 /*
 ** Allocate a new rowid. This is used for "external content" tables when
 ** a NULL value is inserted into the rowid column. The new rowid is allocated