Add file polling alternative to Envoy file-based SDS

*Title*: Add file polling alternative to Envoy file-based SDS

*Description*:
Some kernel features, such as virtiofs, do not support inotify. Because of this, there is currently no way to automatically reload secrets defined via file-based SDS when the secrets are stored on one of these filesystems.

I'd like to request that Envoy support polling these files as an alternative to inotify triggers. This could be handled automatically based on the filesystem that `watched_path` points to, or via explicit user configuration.

[optional *Relevant Links*:]
See https://lwn.net/Articles/874000/ for the most recent attempt at getting this added to the kernel.