xds-failover: adding support for legacy SotW and delta

[adisuissa]

Commit Message: xds-failover: adding support for legacy SotW and delta

Additional Description: This is a breakdown of #34417 to multiple PRs. In this PR we introduce the envoy.restart_features.xds_failover_support runtime flag which defaults to false. We also modify the legacy GrpcMux SotW and Delta implementations to use the GrpcMuxFailover object instead of the GrpcStream object when the runtime flag is set to true.

Next is updating the Unified Sotw/Delta GrpcMux implementation, and plumbing the API through to use the xDS-Failover.

Risk Level: medium if the runtime flag is set to true. Testing: Updated the test cases (parameterized). Docs Changes: N/A Release Notes: N/A Platform Specific Features: N/A

[repokitteh-read-only[bot]]

CC @envoyproxy/runtime-guard-changes: FYI only for changes made to (source/common/runtime/runtime_features.cc).

:cat:

Caused by: https://github.com/envoyproxy/envoy/pull/34437 was opened by adisuissa.

see: more, trace.

[adisuissa]

/assign @htuch

[adisuissa]

@nezdolik can you please take the !Google reviewer role? /assign @nezdolik