feat(grpc-gcp): Add shared fallback state and probing recovery options to GcpFallbackChannel

[kinsaurralde]

1. Moves stat collection used to determine error rates for fallback from GcpFallbackChannel to GcpFallbackState.

2. Adds the ability for the probing function to switch the channel back to the primary channel from fallback if thresholds are met.

This is in preparation of spanner adding a probe and recovery policy. Currently in spanner, GcpFallbackChannel is at the pool level. However for probing and recovery we need each channel to be probed. This shared state allows switching GcpFallbackChannel to be at the channel level (with each object holding 1 directpath and 1 cloudpath) while having the fallback policy move all channels together

Using probes for recovery is an option that will be disabled by default to match current condition.

[gemini-code-assist]

Code Review

This pull request introduces GcpFallbackState to enable coordinated pool-wide failover, recovery, and background task management across multiple channels, along with options for per-channel independent recovery. The review feedback highlights critical issues in the state machine and probing logic: specifically, localInFallbackMode is not properly reset when transitioning out of fallback mode under pool-level recovery, which can corrupt probing statistics on subsequent failovers. Additionally, a potential race condition in probePrimary() could cause localFirstPrimaryProbeSuccessNanos to be read as zero, prematurely satisfying the recovery duration check.