feat: ledger signer support

[xJonathanLEI]

The very first attempt at adding Ledger support. The Ledger support is feature-gated by a ledger feature on both the starknet-signers and starknet crates. It's turned off by default.

The current Ledger app for Starknet is quite limited: it only supports blind signing. Nevertheless, it's a huge step forward compared to software keystores.

This PR adds 2 examples that work as long as you have Ledger plugged in, unlocked, and the Starknet app open. Test with:

cargo run --example ledger_public_key
cargo run --example deploy_account_with_ledger

Major caveats of the current implementation:

1. Confirming the public key on the device's screen is not supported;
2. Higher-level types like Account and AccountFactory still hold the assumption that all signers are non-interactive, and would request signatures even just for fee estimation; this results in excessive signing prompts on the Ledger device; (update: implemented in feat: signer interactivity #617)
3. The EIP-2645 path is difficult to write correctly without copy pasting; maybe we should add a type that encapsulates this and converts into DerivationPath.

That said, if anyone wants to start using it from this PR, they totally can and it's 100% functional :)

[xJonathanLEI]

Pathfinder bug is causing test failures: eqlabs/pathfinder#2085