fix network argument to be a string and not a list

[jakubgs]

Tried using TOML config format and encountered this unexpected error:

Failed to load secondary sources: 'nimbus-execution-client.toml(17, 11) expected '"["''

[jangko]

The reason it is a list is nimbus can accept:

1. --network:hoodi
2. --network:1
3. --network:hoodi --network:123
4. --network:"/path/to/genesis" --network:123

The 3rd & 4th version allow nimbus to override the numerical network id.
What need to be fixed is the TOML parser, but right now nim-toml-serialization is less flexible and missing a crucial feature to parse dynamic field type like nim-json-serialization.

[jakubgs]

If overriding network ID is required why not have separate --network-id flag to make functionality explicit and flag parsing simpler?

Also, why would overriding the network ID be desirable in the first place?
And why providing genesis file shouldn't be a separate flag as well?

[jangko]

In the past nimbus have --network-id and --custom-network, they do not make parsing simpler. That's why --network-id is removed, and --custom-network been deprecated.

And why overriding network ID is needed? Looks like geth genesis file format is very popular among client implementer. But it does not contains network ID. It force chain ID to be used as network ID which is not always true. override network ID is a clean mechanism when client user want a different network ID from chain ID. (So it's an external/historical complexity)

And why providing genesis file shouldn't be a separate flag as well? both genesis file and built in named network are the same thing. You can choose a built in named network such as hoodi or a genesis.json of hoodi and it will be the same configuration for your client.

For now, isn't using bracket for your network field in your TOML file is simpler than try to fix the parser?