Add grpc authentication to maxengine_server

[bvandermoon]

Description

Adds grpc_tls_certificate_path and grpc_tls_private_key_path configuration options to InferenceServer and updates maxengine_server.py to support secure gRPC TLS connections when configured.

• Enables users to configure gRPC server credentials with TLS certificates and private keys for secure serving environments.
• Defaults to insecure credentials when TLS paths are not provided, preserving seamless local unit testing and development workflows.

FIXES: b/510375382

Tests

Ran configuration unit tests on TPU VM:

pytest tests/unit/configs_test.py tests/unit/configs_value_test.py

All tests passed successfully.

Checklist

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• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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