Private keys as string in v3

[kanej]

When a user provides a private key as a string in HHv2, the 0x is optional.
In v3 we should trim the 0x everywhere.

[schaable]

@alcuadrado Does this mean the user can input private keys (only the private keys) in the accounts section of the network config, either with or without the "0x" prefix, and we internally resolve it to the prefixed version?

[schaable]

Updating the issue for future reference: Users can input the private keys in networkConfig.accounts either with or without the "0x" prefix. Internally, they are normalized to a "0x" prefixed string.