Support SSH Agent for public key authentication when connecting to a remote host

[yanjiew1]

Currently, Theia does not utilize the SSH agent for public key authentication. Instead, it reads the private key directly from the ~/.ssh directory. If the private key is password-protected and the SSH agent is not used, users are required to manually enter the passphrase each time they connect to a remote host.

Feature Description:

If the SSH_AUTH_SOCK environment variable is set, Theia should connect to the SSH agent to handle public key authentication, rather than relying on the private key in ~/.ssh.