appendSignMultisigTransaction takes a multisig transaction blob, and appends our signature to it.
While we could derive public key preimagery from the partially-signed multisig transaction,
we ask the caller to pass it back in, to ensure they know what they are signing.
Parameters
multisigTxnBlob: Uint8Array
an encoded multisig txn. Supports non-payment txn types.
appendSignMultisigTransaction takes a multisig transaction blob, and appends our signature to it. While we could derive public key preimagery from the partially-signed multisig transaction, we ask the caller to pass it back in, to ensure they know what they are signing.