Add `--hd-path` flag to `keys add --secure-store`

[fnando]

Summary

stellar keys generate --secure-store accepts a --hd-path flag, but stellar keys add --secure-store does not. The add command always implicitly uses HD path 0 (or whatever the default ends up being). The two commands should support the same set of derivation paths so users can import a seed phrase and immediately use a non-default account index.

Context

Spotted by @leighmcculloch during review of #2533:

• Cache secure-store public key in identity files. #2533 (comment)
• Cache secure-store public key in identity files. #2533 (comment)

In #2533 the call site in cmd/soroban-cli/src/commands/keys/add.rs passes None for hd_path because the flag isn't wired up yet — that's the placeholder this issue would replace.

Proposed change

• Add a --hd-path <usize> flag to keys add (mirroring the one on keys generate at cmd/soroban-cli/src/commands/keys/generate.rs:55).
• Thread it through to the secure_store::save_secret(...) call so the derived public key cached in the identity TOML matches the requested path.
• Document the flag in the help text consistently with generate.

[2411015047-afk]

Hi! I can add the --hd-path flag to stellar keys add --secure-store. This is a focused Rust CLI change to match the existing support in stellar keys generate --secure-store. Let me know if you'd like me to implement this as a paid contribution.