Support piping keys into `stellar keys add`

[leighmcculloch]

What problem does your feature solve?

stellar keys add currently requires interactive input to provide the secret key. This makes it difficult to use in scripts, CI/CD pipelines, and automated environments where keys need to be added non-interactively.

What would you like to see?

Support reading the secret key from stdin so that keys can be piped in. For example:

echo 'S...' | stellar keys add my-key

This would make the CLI more script-friendly and easier to use in automated workflows, containers, and other non-interactive environments.

What alternatives are there?

• Manually create or mount the identity/config files that stellar keys add would normally generate, bypassing the command entirely.
• Use expect or similar tools to automate the interactive prompt, which is fragile and harder to maintain.