ext_authz: allow receiving dynamic metadata from gRPC authorization server

[erikbos]

Title: ext_authz: enable receiving dynamic metadata from gRPC authorization server

Description:
I've got a custom authorization server which looks up some additional data for each request that I like to carry forward to other filters, but not upstream.

Example use cases for setting metadata:

1. Setting dynamic rate limit value, authorization server has bespoke logic to determine rate limiting setting for request (leverages Proposal: support for dynamic rate limits #11595)
2. Passing on internal authorization server fields which should end up in the access log (e.g. customerId, "rate limiter plan")

Relevant links
I think this might be the opposite of feature request #7699 (Fixed with PR #7818)

[mattklein123]

cc @dio @gsagula

[stale]

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 other activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically closed because it has not had activity in the last 37 days. If this issue is still valid, please ping a maintainer and ask them to label it as "help wanted". Thank you for your contributions.

[erikbos]

@Pchelolo FYI