pantalaimon: Add the ability to index only encrypted rooms.

Since some public rooms can be quite large downloading the room history
for such rooms can be quite resource intensive.

Replicating the whole room history locally for such rooms might be
undesirable. This patch adds the ability to only download room history
and index messages for encrypted rooms.

Do note that the search API supports searching in multiple rooms, thus
if the indexing is restricted to encrypted rooms a search across
multiple rooms will need to make a search request to the server as well
as a local search. This mode is currently unsupported.

docs/man/pantalaimon.5

@@ -66,6 +66,13 @@ Homeserver, note that this will make the search much slower. If this is set to
 .Nm pantalaimon
 will not make any additional HTTP requests and will leave some data fields in
 the search response empty. Defaults to "Off".
+.It Cm IndexEncryptedOnly
+A configuration option to decide if
+.Nm pantalaimon
+should fetch the history for
+unencrytped rooms as well as for encrypted ones. If True, only the history for
+encrypted rooms is fetched and indexed. Search requests for non-encrypted
+rooms are forwarded to the Homeserver.
 .El
 .Pp
 Aditional to the homeserver section a special section with the name