BEvm: separate `transaction` and `deploy` methods to make authorization more flexible

[ineiti]

Currently, the bevm ByzCoin contract supports two methods:

• credit to credit an EVM account
• transaction to perform an EVM transaction

The latter one comprises both EVM transactions to deploy a new contract and transactions to execute a state-changing method on an already deployed contract (as they are very similar from the EVM point of view).
In order to allow for more flexible authorization management through DARCs, these two operations should be separated into deploy and transaction so that an identity can be granted the right to execute operations on already deployed EVM contracts, but not to deploy new ones.