Versioning and updates for proxy config #277
Description
When implementing proxy config format for our use cases I run into a challenge with versioning and policy refresh.
That is, client needs to constantly make a HEAD request for proxy config to see if it was updated or not. And in some cases (VPN-like remote access) configuration can change rapidly.
I think it would be great to include a "version" field (could be an opaque string) and propose a HTTP header for MASQUE proxies to include in their responses that would signal the latest proxy configuration version. This way client knows when its proxy config is outdated and needs to be refreshed.
On top of that we could specify that implementors should use Expires header as per RFC9111 for client to periodically refresh proxy configuration.