[ecash-wallet] Implement MultisigWallet with ECDSA multisig support

Summary:
This feature allows users to execute most wallet actions, including token actions, in a multisig wallet. For now, ECDSA-only, which is easier to implement. We also do not support HD or chained actions.

The first anticipated use case is multisig token actions.

We follow electrum-abc convention of lexicographic ordering of pubkeys in ecash-wallet. So, users creating multisig wallets with the same pks will always get the same p2sh address. ecash-lib will accept any ordering.

Compared to non-multisig wallets,

• Multisig wallets do not auto-update the utxo set on build, since the utxo is not expected to be used until everyone signs
• Multisig wallets do not attempt to retry on utxo set failures, since we cannot rebuild the tx without all parties signing again

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D19777