s2a: use AdvancedTls.

[rmehta19]

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Fixes #<issue_number_goes_here> ☕️

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Summary of Changes

Hello @rmehta19, I'm Gemini Code Assist[^1]! 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 refactors the S2A (Secure Session Access) channel credential provisioning within the InstantiatingGrpcChannelProvider to adopt gRPC's AdvancedTls capabilities. The primary goal is to improve the robustness and maintainability of TLS certificate and private key management by enabling scheduled, automatic updates for both identity and trust credentials, thereby enhancing the security posture and operational flexibility of the gRPC channels.

Highlights

• Advanced TLS Implementation: The pull request transitions the S2A channel credential management to utilize gRPC's AdvancedTlsX509KeyManager and AdvancedTlsX509TrustManager for enhanced TLS handling.
• Scheduled Credential Updates: Introduced scheduled updates for both identity (key) and trust credentials, allowing for dynamic refreshing of certificates and private keys at a specified interval (currently 1 hour).
• Dedicated Executor Services: New ScheduledExecutorService instances are created specifically for managing the periodic updates of the key and trust managers, ensuring these operations run in dedicated daemon threads.
• Exception Handling: The createMtlsToS2AChannelCredentials method signature has been updated to include GeneralSecurityException to account for potential security-related issues during credential management.

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[rmehta19]

@ejona86, @gtcooke94