nostrdb/blocks: add total_size

Fix this mistake that we have with ndb_notes where we don't know the
total size of the object

Signed-off-by: William Casarin <jb55@jb55.com>

nostrdb/src/block.c

@@ -196,3 +196,8 @@ uint32_t ndb_str_block_len(struct ndb_str_block *str_block) {
 struct nostr_bech32 *ndb_bech32_block(struct ndb_block *block) {
 	return &block->block.mention_bech32.bech32;
 }
+
+// total size including padding
+size_t ndb_blocks_total_size(struct ndb_blocks *blocks) {
+	return blocks->total_size;
+}