ecds: support for upstream network filter

[ohadvano]

Additional Description: Support of upstream network filter with ECDS. Resolves #14696.
Risk Level: low
Testing: Integration tests
Docs Changes: ECDS docs
Release Notes: None
Platform Specific Features: None
cc @htuch

[repokitteh-read-only]

As a reminder, PRs marked as draft will not be automatically assigned reviewers,
or be handled by maintainer-oncall triage.

Please mark your PR as ready when you want it to be reviewed!

🐱

Caused by: #28860 was opened by ohadvano.

see: more, trace.

[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 @adisuissa
CC @envoyproxy/api-watchers: FYI only for changes made to (api/envoy/|docs/root/api-docs/).

🐱

Caused by: #28860 was opened by ohadvano.

see: more, trace.

[ohadvano]

Moving back to draft as there are tests failing which don't reproduce locally, will try to resolve this