feat(spanner): Add E2E fallback to the spanner client.

[kinsaurralde]

Adding E2E fallback will allow spanner clients to fallback to cloudpath when directpath is unavailable

[gemini-code-assist]

Summary of Changes

Hello @kinsaurralde, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a critical reliability feature for the Spanner client by implementing an end-to-end fallback mechanism. This allows the client to seamlessly transition from using "directpath" to "cloudpath" when "directpath" becomes unavailable, significantly enhancing the client's resilience and ensuring continuous operation without manual intervention.

Highlights

• E2E Fallback Mechanism: Implemented an end-to-end fallback mechanism for the Spanner client, enabling it to switch from "directpath" to "cloudpath" if "directpath" is unavailable.
• FallbackWrapper Introduction: Introduced a new fallbackWrapper struct that encapsulates grpcgcp.GCPFallback along with primary and fallback connection pools, managing their lifecycle.
• Conditional Fallback Activation: Modified the newClientWithConfig function to conditionally enable the fallback logic based on the GOOGLE_SPANNER_ENABLE_GCP_FALLBACK environment variable and isDirectPathEnabled.
• Dependency Update: Updated the grpc-gcp-go/grpcgcp dependency to version v1.6.0 to support the new fallback features.
• Session Client Integration: Updated the sessionClient.nextClient method to correctly handle the newly introduced fallbackWrapper type for connection management.

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[gemini-code-assist]

Code Review

This pull request introduces a fallback mechanism for the Spanner client, allowing it to switch from DirectPath to CloudPath when DirectPath is unavailable. This is a valuable feature. However, the current implementation has several critical issues, including potential panics from nil pointer dereferences, resource leaks due to improper connection handling in error paths, and incorrect error propagation. I've provided specific comments and suggestions to address these issues to ensure the stability and correctness of the new fallback logic.

[rahul2393]

@kinsaurralde can you please fix the conflict?