ext_authz: add knob enforce_response_header_limits

[antoniovleonti]

Commit Message: ext_authz: add knob enforce_response_header_limits
Additional Description:

This PR adds a config knob to enable / disable the behavior of the ext authz filter dropping headers after the response header map limits have been reached.

This allows you to enable the behavior if you are using an untrusted external auth service (providing protection against adding too many headers) or disable the behavior if you trust the service.

Risk Level: low
Testing: unit tests added
Release Notes: added

[repokitteh-read-only]

CC @envoyproxy/api-shepherds: Your approval is needed for changes made to (api/envoy/|docs/root/api-docs/).
envoyproxy/api-shepherds assignee is @mattklein123
CC @envoyproxy/api-watchers: FYI only for changes made to (api/envoy/|docs/root/api-docs/).

🐱

Caused by: #41742 was opened by antoniovleonti.

see: more, trace.

[antoniovleonti]

should this be a feature flag instead of a config? it seems like I am adding a config that is insecure by default to guard a security enhancement

[antoniovleonti]

should this be a feature flag instead of a config? it seems like I am adding a config that is insecure by default to guard a security enhancement

Counter argument: in a use case where the ext authz service is trusted it may always be valid to disable this check (since you know the CheckResponse won't contain an "unreasonable" amount of response headers)

OTOH there is no equivalent knob in ext_proc nor in the request path.

[antoniovleonti]

/assign @tyxia