xDS-failover: attempt reconnection to primary after disconnected from…

… failover (#35591)

Prior to this PR, Envoy would stick to either the primary or failover xDS source, once it got a response from one.
This PR changes the behavior by allowing Envoy to attempt reconnecting to the primary source, after being disconnected from the failover source.

Risk Level: low - xDS-failover is disabled by default.
Testing: Added unit and integration tests.
Docs Changes: N/A - docs will be created in a subsequent PR.
Release Notes: N/A - no release note yet as this is WIP.
Platform Specific Features: N/A

Signed-off-by: Adi Suissa-Peleg <[email protected]>