build: moving filesystem subscription (#26521)

Risk Level: low
Testing: n/a
Docs Changes: n/a
Release Notes: inline

#26450
Signed-off-by: Alyssa Wilk <alyssar@chromium.org>

Mirrored from https://github.com/envoyproxy/envoy @ a75b99df543f7ccb9f562fd6abcd3173eaa445b5

envoy/config/core/v3/config_source.proto

@@ -285,3 +285,11 @@ message ExtensionConfigSource {
 // A placeholder proto for the registration for the REST subscription implementation.
 message RestSubscription {
 }
+
+// A placeholder proto for the registration for the filesystem subscription implementation.
+message FilesystemSubscription {
+}
+
+// A placeholder proto for the registration for the filesystem collection subscription implementation.
+message FilesystemCollectionSubscription {
+}