Support restoring multisig wallets from seed in monero-wallet-rpc
This issue requests restoring multisig wallets from seed in monero-wallet-rpc.

Currently `restore_deterministic_wallet` returns an error "invalid mnemonic" with a multisig seed.

monero-wallet-cli supports restoring from seed using the `--restore-from-seed` startup flag, but ideally support can be added to monero-wallet-rpc without using a startup flag.