feat: allow stellar key secrets to be input via pipe

[mootz12]

What

Allow secret keys to be input via a pipe.

Example:

echo "S..." | stellar keys add my-new-key

Why

Better support for CI environments to safely create keys with the CLI.

Closes: #2388

Known limitations

None

[Copilot]

Pull request overview

This PR adds support for piping secret keys via stdin to the stellar keys add command, enabling better integration with CI/CD pipelines and automated workflows. The implementation uses Rust's IsTerminal trait to detect whether stdin is a terminal and branches between interactive prompting and non-interactive stdin reading accordingly.

Changes:

• Modified read_password function to detect terminal vs non-interactive mode and read from stdin when piped
• Added integration test to verify secret keys can be successfully added via stdin
• Cargo.lock updated with removal of soroban-spec-json dependency

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 1 comment.

File	Description
cmd/soroban-cli/src/commands/keys/add.rs	Adds terminal detection and stdin reading logic to read_password function to support piped input
cmd/crates/soroban-test/tests/it/config.rs	Adds integration test verifying secret keys can be added via stdin piping
Cargo.lock	Removes soroban-spec-json package dependency (appears unrelated to main feature)