allow HTTP filters to be marked as optional

[markdroth]

#14982 added new fields to the API to allow HTTP filters to be marked as optional -- i.e., if a filter is not known to a client, the client will ignore that particular filter instead of NACKing the entire config. gRPC is implementing this now, and we'd like Envoy to have the same behavior.

CC @htuch

[github-actions]

This issue has been automatically marked as stale because it has not had activity in the last 30 days. It will be closed in the next 7 days unless it is tagged "help wanted" or "no stalebot" or other activity occurs. Thank you for your contributions.

[markdroth]

@htuch, can we tag this so that it doesn't go stale? Thanks!