feat: add TLS support for gRPC connections to Dex

[norajam]

This PR adds support for TLS-secured gRPC connections between the gateway and Dex, enabling secure deployments where Dex and the gateway run on different hosts.

Why this matters

Previously, the gateway could only connect to Dex via insecure gRPC, which limited deployment options. With TLS support, Dex and the gateway can now be deployed separately across different hosts or networks while maintaining secure communication.

What's new

The gateway can now establish secure gRPC connections to Dex using mTLS (mutual TLS). This is configured through the dexGRPCClient section in the config file, where you can specify:

• Client certificate and key for authentication
• CA certificate for validating the Dex server

If TLS configuration isn't provided, the gateway falls back to insecure credentials, maintaining backward compatibility with existing setups.

Testing

Tested with both TLS-enabled and insecure configurations to ensure the feature works correctly and doesn't break existing deployments.