Converge to unified gRPC mux

[adisuissa]

Converge to unified gRPC mux

Description:
Envoy's support for the State-of-the-World and Delta-xDS gRPC muxes started as 2 different implementations.
There's been an effort (#17352) to converge the 2 code-bases into a single one - unified gRPC mux - that is currently under a runtime guard envoy_reloadable_features_unified_mux and defaults to false. Before converting the runtime flag to true, Envoy needs to ensure that doing so won't result in a breaking behavior.

Known differences will be tracked in this issue:

1. A request for a child resource will always be sent after receiving the parent resource in the non-unified SotW implementation.
   Example: Say a cluster configured to use EDS is received using CDS. In the non-unified SotW implementation, Envoy will always send an EDS request for that resource, even when receiving an update to a known cluster. In the (nun-unified) delta-xDS implementation, and in the unified gRPC implementations (SotW and delta-xDS), Envoy will only send a request on the first request for the EDS resource.
   A server may erroneously depend on receiving that request in order to send an update to the Envoy.

AIs:

• Ensure that Envoy behaves as expected without receiving the response for a known resource. Known issue with EDS (xds: Consistency while handling CDS updates and EDS #13009, addressed in Using cached EDS resources #26749), and need to validate that other xDS subscriptions work properly.
• Ensure that all gRPC-based tests run with the unified and non-unified mux implementation.

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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 "no stalebot" or other activity occurs. Thank you for your contributions.