Allow rlp/era file import to import from custom network
With the addition of era file import, the rlp block import become cannot import from custom network.
This is because the era file import currently only support built in network configuration(mainnet, holesky, sepolia).
And rlp import is processed after era file checks.

We can allow era file import to support custom network. or move rlp import to a new cli switch e.g. `rlp-import`.