ethers-signers: support multiple Ledger instantiations

[joshieDo]

ref foundry-rs/foundry#2790

ethers-rs/ethers-signers/src/ledger/app.rs

Lines 34 to 35 in 2e9ee95

	impl LedgerEthereum {
	/// Instantiate the application by acquiring a lock on the ledger device.

It would help if we could have multiple Ledger instantiations (from multiple derivations) without fighting for the lock. Alternatively, we could have a wrapper in foundry, but I feel like it's better suited here.