Convert sequencer private key type to k256 private key from vec<u8> before mainnet

[ercecan]

In order to keep the backwards compatibility after #1798 we had to keep the sequencer private key as a vector because we convert it to dalek or k256 private key when needed. Before mainnet in order to reduce these unnecessary conversions, since we will only be using k256, we can just use k256 private key concrete type