Add ability to sign Safe transactions using Ledger, Trezor and Lattice1 via Frame.

Tested on Ethereum Mainnet with Ledger Nano X.

New API:

• get_signer()
• sign_with_frame(safe_tx)
• post_signature(safe_tx, signature_
• pending_transactions
• confirmations_to_signatures(confirmations)
• execute_transaction

Some specific combinations I want tested:

• safe on any network but eth mainnet which has a ledger as owner (ledger doesn't support eip155 and i want to see if i implemented the fix for it correctly)
• safe on any network with any trezor model as owner
• safe on any network with lattice1 as owner