nostrdb: Implement nip50 fulltext searching

This adds support for nip50 fulltext searches. This allows you to use
the nostrdb query interface for executing fulltext searches instead of
the typical `ndb_text_search` api. The benefits of this include a
standardized query interface that also further filters on other fields
in the filter.

Changelog-Added: Add nip50 search filters and queries
Signed-off-by: William Casarin <jb55@jb55.com>

nostrdb/src/nostrdb.h

@@ -314,6 +314,10 @@ struct ndb_text_search_key
 struct ndb_text_search_result {
 	struct ndb_text_search_key key;
 	int prefix_chars;
+
+	// This is only set if we passed a filter for nip50 searches
+	struct ndb_note *note;
+	uint64_t note_size;
 };
 
 struct ndb_text_search_results {