Add --ledger to stellar keys add.

[fnando]

What

Also persist public key and hd path to a config file, so we don't need to interact with the hardware on read-only operations.

Why

To match and have a uniform experience all around.

Known limitations

n/a

[Copilot]

Pull request overview

This PR extends stellar keys add with a dedicated --ledger flag and updates config serialization to persist a Ledger-derived public key (and optional --hd-path) so read-only operations can work without a connected device.

Changes:

• Add stellar keys add --ledger (async) to derive and persist a Ledger-backed identity (cached public_key + optional hd_path).
• Update Secret::Ledger persistence format to include hardware, cached public_key, and optional hd_path, plus unit tests for TOML round-trips.
• Adjust a few parsing/error-path tests and avoid attempting Ledger HID use in contract signer resolution.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
FULL_HELP_DOCS.md	Documents the new --ledger option for stellar keys add.
cmd/soroban-cli/src/config/secret.rs	Redefines Secret::Ledger to persist cached public_key/hd_path and adds related tests; updates key derivation/signing behavior.
cmd/soroban-cli/src/config/key.rs	Updates a parsing test expectation now that "ledger" is no longer parsed as a secret literal.
cmd/soroban-cli/src/config/address.rs	Removes the special-case rejection of the identity name "ledger".
cmd/soroban-cli/src/commands/message/verify.rs	Updates a test expectation for "ledger" handling in public key resolution.
cmd/soroban-cli/src/commands/keys/mod.rs	Makes keys add invocation await the now-async implementation.
cmd/soroban-cli/src/commands/keys/add.rs	Implements --ledger flow (derivation + persistence), adds clap conflict tests, and converts affected tests to async.
cmd/soroban-cli/src/commands/contract/arg_parsing.rs	Skips resolving a Ledger signer in the co-signer path to avoid HID transport conflicts.

[leighmcculloch]

One concern, but defer to you on it.