Adds new `subjects` config field to restrict subjects accepted from a `JwtProvider` partially implementing #31455
Risk Level: Low
Testing: Unit testing
Docs Changes: Added `subjects` description inline in proto.
Release Notes: Attached
Optional [API Considerations](https://github.com/envoyproxy/envoy/blob/main/api/review_checklist.md): Feature is opt in, without specifying the config, there's no behavior change.
Signed-off-by: Matthew Jones <mattjo@squareup.com>
Mirrored from https://github.com/envoyproxy/envoy @ 08231e383fc3fb1c3bb207774d8295995759552a