Support recursive subdirectories for file-provider

*Description*:

Currently, the `paths` in file-provider does not support watch any recursive subdirectories:

https://github.com/envoyproxy/gateway/blob/2c986175211df64a269e19111b9ed480646c285a/api/v1alpha1/envoygateway_types.go#L308

It has been noted that each backend service related configs can be put in separate directory, but instead of watching all these directories, we can treat them as subdirectories of a root dir, and only watching this root dir.

[optional *Relevant Links*:]

Discussed in https://envoyproxy.slack.com/archives/C03E6NHLESV/p1731083390060779?thread_ts=1731035129.484099&cid=C03E6NHLESV