Allow FileSystemBufferFilter to be bypassed without creating a thread (#21852)

Signed-off-by: Raven Black <ravenblack@dropbox.com>

Mirrored from https://github.com/envoyproxy/envoy @ 4b05b1e6052c5b62e5447a0696492fc1b7a4e828

envoy/extensions/filters/http/file_system_buffer/v3/file_system_buffer.proto

@@ -110,7 +110,8 @@ message StreamConfig {
 message FileSystemBufferFilterConfig {
   // A configuration for an AsyncFileManager.
   //
-  // If unset in route, vhost and listener, an exception will be thrown.
+  // If unset in route, vhost and listener, and the behavior is not ``bypass``
+  // in both directions, an Internal Server Error response will be sent.
   common.async_files.v3.AsyncFileManagerConfig manager_config = 1;
 
   // An optional path to which the unlinked files should be written - this may