migrator: Add an escape hatch to allow arbitrary config options

Previously only SQL settings and the 'path' for internal storage could
be set, and only for SQL and internal storage.

input {
	type = "whatever";
	config = {
		whatever_foobar = "something"
	}
}

tools/migration/prosody-migrator.lua

@@ -168,6 +168,11 @@ local function prepare_config(host, conf)
 	elseif conf.type == "sql" then
 		cm.set(host, "sql", conf);
 	end
+	if type(conf.config) == "table" then
+		for option, value in pairs(conf.config) do
+			cm.set(host, option, value);
+		end
+	end
 end
 
 local function get_driver(host, conf)