xds-failover: adding main logic

[adisuissa]

Commit Message: xds-failover: adding main logic Additional Description: Adding the main logic for the xDS-failover support. It currently supports a primary and failover sources. Initially, the manager attempts to connect to the primary twice, and if both fail, alternates between the failover and the primary until one is successfully connected to. Once a connection is made (a response arrives) from either source, the failover manager will stick to that source.

Risk Level: low - not plumbed into any GrpcMux object yet. Testing: Added unit test. Docs Changes: N/A Release Notes: N/A Platform Specific Features: N/A

[repokitteh-read-only[bot]]

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:cat:

Caused by: https://github.com/envoyproxy/envoy/pull/34215 was opened by adisuissa.

see: more, trace.

[adisuissa]

Harvey PTAL. LMK if you want more context (I've got an internal integration test the exercises this code). /assign @htuch

[adisuissa]

Assigning @soulxu as !Google maintainer reviewer. /assign @soulxu

[adisuissa]

Unassigning @soulxu as he is OOO this week. @wbpcode can you please take a ~Google pass at this PR?

[adisuissa]

@nezdolik will you have some bandwidth to review this PR as a !Google reviewer? Thanks in advance!