… CDS SotW (#41117)

Prior to this PR, an Envoy that used (old) ADS over SotW to fetch all
the clusters and a xDS-TP-config-based source to fetch OD-CDS would see
the OD-CDS clusters removed once the ADS update arrives.
This PR, changes the SotW CDS wildcard update, and adds tracking to see
which resources are fetched from that source. The tracking is only used
if both (old) ADS and an xDS-TP based config is configured.
This implementation is currently only for CDS, but will need to be part
of the (future) xDS-Cache layer.

Continuation of #40988.

Risk Level: low - should only impact cases that use xDS-TP based configs
(guarded by a false by default runtime guard).
Testing: Added both unit and integration test.
Docs Changes: N/A
Release Notes: N/A
Platform Specific Features: N/A

---------

Signed-off-by: Adi Suissa-Peleg <adip@google.com>

…0988)

Commit Message: xds-federation: introduce OD-CDS xds-federation support
Additional Description:

This PR introduces OD-CDS xds-Federation support. The implementation
uses a new OD-CDS subscription management on the main thread, that is
simpler than the former (issues subscriptions directly using the
xDS-manager subscription function), and doesn't need to maintain the
state of whether the first request was sent or not.
It does so by allowing each subscription be a singleton subscription on
the component side (the gRPC-mux to the xds-Federation server will
ensure that a single subscription message is sent to the server). This
approach handles some bugs that were detected for the non-xDS-TP based
OD-CDS implementation.
The plan is first to use this implementation for xDS-TP based resources,
and when it is validated in production, move the ADS and the
specific-config-source paths to use the new implementation/model.

Apologies for the large PR, but the number of tests that was needed here
is quite large.

Risk Level: low - only for xDS-TP based config sources and it is guarded
by the
`envoy.reloadable_features.xdstp_based_config_singleton_subscriptions`
runtime guard which is currently disabled.
Testing: Added both unit-tests and integration tests
Docs Changes: N/A (until the entire xDS-TP based config sources support
is done).
Release Notes: N/A (until the entire xDS-TP based config sources support
is done).
Platform Specific Features: N/A

---------

Signed-off-by: Adi Suissa-Peleg <adip@google.com>
Signed-off-by: Misha Badov <mbadov@google.com>

… CDS SotW (envoyproxy#41117)

Prior to this PR, an Envoy that used (old) ADS over SotW to fetch all
the clusters and a xDS-TP-config-based source to fetch OD-CDS would see
the OD-CDS clusters removed once the ADS update arrives.
This PR, changes the SotW CDS wildcard update, and adds tracking to see
which resources are fetched from that source. The tracking is only used
if both (old) ADS and an xDS-TP based config is configured.
This implementation is currently only for CDS, but will need to be part
of the (future) xDS-Cache layer.

Continuation of envoyproxy#40988.

Risk Level: low - should only impact cases that use xDS-TP based configs
(guarded by a false by default runtime guard).
Testing: Added both unit and integration test.
Docs Changes: N/A
Release Notes: N/A
Platform Specific Features: N/A

---------

Signed-off-by: Adi Suissa-Peleg <adip@google.com>
Signed-off-by: Misha Badov <mbadov@google.com>