Rename `RewardAccount` to `Account`

With CIP-159 it will no longer be accurate to call it "reward account", since it is no longer just about rewards.
We will also need to rename `raCredential` to `aCredential` and `raNetwork` to `aNetwork`

As usual we should add `RewardAccount{raNetwork, raCredential}` pattern synonym with deprecation. This applies to all other helpers that will need to be renamed due to this rebranding.