Support for `stellar keys address --ledger`

[fnando]

Summary

stellar keys address currently requires a named identity that resolves to a local keypair or stored key. There is no --ledger flag, so it is not possible to retrieve the public address of a Ledger hardware wallet account directly from the CLI.

Motivation

Knowing the address derived from a Ledger device is a fundamental operation — needed before funding, before sharing an address with a counterparty, and before scripting any workflow that involves a hardware wallet. Without this, users must open a separate tool (e.g. Ledger Live or a web wallet) just to look up their own address, which breaks the CLI-first development experience.

Proposed solution

Add a --ledger flag to stellar keys address that:

1. Connects to the attached Ledger device and derives the public key for account index 0 (i.e. m/44'/148'/0') by default.
2. Prints the corresponding Stellar address to stdout.
3. Respects --hd-path <N> when provided, where N is the integer account index used to derive the address at m/44'/148'/N'.

Desired usage:

Print the address for the default Ledger account (index 0):
  stellar keys address --ledger

Print the address for a specific account index:
  stellar keys address --ledger --hd-path 1

Acceptance criteria

• stellar keys address --ledger connects to a Ledger device, derives the public key at index 0, and prints the corresponding Stellar address to stdout.
• stellar keys address --ledger --hd-path N prints the address derived at the given integer index.
• A clear error is shown when no Ledger device is connected or the Stellar app is not open on the device.