Extend SSH key export script to support multiple SSH key types

[josephmilla]

This pull request enhances the existing SSH key export script to support a broader range of SSH key types. The key updates include:

• Dynamic SSH key detection: The script now automatically detects and exports all public SSH keys (*.pub) in the .ssh directory, supporting multiple key types such as RSA, Ed25519, ECDSA, and DSA without the need for hardcoding specific key filenames.
• Loop through key files: The script iterates through all available public key files in the user’s .ssh directory, ensuring flexibility and future-proofing for any new SSH key types.
• Known hosts check: The script retains its functionality to check and output the contents of the known_hosts file.
• Improved error handling: Ensures the script exits early if the current user is not valid or logged in.

This enhancement simplifies managing SSH keys across different environments by automatically handling various key types.

Testing:

$ shellcheck export_ssh_keys.sh