feat: Auto-generate private key if missing for gssh build

[DevonTM]

Summary
This PR improves the usability of the gssh build of tiny-frpc by removing its dependency on pre-existing SSH key files. Previously, gssh build would attempt to read a private key from $HOME/.ssh/id_rsa, which could fail in minimal or embedded environments where:

• OpenSSH or other key generation tools are not installed
• No user-specific .ssh directory exists
• No pre-generated key has been provided

What's changed
When using the gssh build, if no private key (id_ed25519) is found:

• The client will automatically generate an ED25519 key pair
• The keys are stored in the same directory as the tiny-frpc executable
• The gssh build now reads the private key from id_ed25519 in the same directory as the executable, instead of from $HOME/.ssh/id_rsa

Benefits

• Fully self-contained operation in minimal environments (e.g. OpenWRT, containers, custom init systems)
• No requirement for external key generation tools
• Secure by default with ED25519
• Simplifies deployment and scripting

Notes

• Only affects the gssh build mode
• Does not interfere with existing manually-provided keys
• ED25519 was chosen due to its smaller size, faster generation, and strong security