feat: SSHKeyContentResponse, SecretStr, authorized_keys options, and tests

[mkitti]

• Add custom SSHKeyContentResponse that wipes memory after sending content
• Use pydantic.SecretStr to guard ssh private key password
• Add restrict keyword to authorized_keys options
• Add fileglancer to authorized_keys comment
• Add tests for SSH key handling

TODO:

• Only manage authorized keys which have fileglancer in the comment
• Consider using StreamingResponse to further avoid middleware buffers that contain the entire key