Title: ext_authz: allow sending dynamic metadata as context

Description:
AFAICT from the docs & code, the ext_authz attribute context can take "static metadata" only. However, there are certain use-cases where you want to use some metadata passed from one filter and send it to the ext_authz service as context. My specific use-case is using the result of the jwt_authn filter via the payload_in_metadata flag to pass validated JWT information to the ext_authz filter.

Relevant Links:
Not entirely unrelated to #7272