custom network support in nrpc

[advaita-saha]

This is needed while testing and debugging for devnet-6, seems like a nice to have add-on

[tersec]

The UX on this is, I'm not sure -- it seems odd to have this situation where the --network parameter knows it can't find a built-in one and then one has to use this --custom-network-file (as /path/to/metadata.yaml, distinct from --custom-network, which is as /path/to/genesis.json) parameter.

They're in the same directories for all the EF-supported networks:

• https://github.com/eth-clients/mainnet/tree/main/metadata
• https://github.com/eth-clients/sepolia/tree/main/metadata
• https://github.com/eth-clients/holesky/tree/main/metadata
• https://github.com/eth-clients/hoodi/tree/main/metadata

So while it's fine to document that these files are different (metadata.yaml vs genesis.json) the actual path is the same and having two different command-line options for basically the same thing is odd.

There's a separate question of, should it be a --custom-network at all or a nimbus-eth2-style implied check in --network, but at least for this PR, it seems ok to at least avoid creation of similar-but-not-identical --custom-network-file.