This is a necessary change to the meta transaction signature scheme to
address security related concerns with the original proposal.

Meta transactions introduce `DelegateAction` and
`SignedDelegateAction` where the signature key is an account key.
Transactions are also signed by account keys.

We don't want to have the possibility that the borsh serialization of a
`Transaction` and a `DelegateAction` end up with the same binary
representation. In that case it would be indistinguishable whether the
account owner signed the Transaction or the DelegateAction, which
opens the door to various security vulnerabilities.

There is the draft NEP-461 to solve this in a general way. To push
forward with meta transaction, we will use that proposed schema.
Regardless of whether the standard gets accepted or not, at least this
solves the problem for meta transactions and makes it possible to add
more types of messages later that are also signed with account keys.

This is not a protocol change. It only affects meta transaction which
have not been stabilized yet.

- add link to NEP-461
- rename `get_hash` to `get_nep461_hash`