Make NetworkConfig serializable

[onbjerg]

Describe the feature

The idea of the Config struct used in the CLI is that it is a thin wrapper around existing config structs to the extent possible to lower maintenance, i.e. instead of having to add new config in 2 places, you just add it in one.

Currently though, it is not possible to serialize or deserialize the NetworkConfig struct (or NetworkConfigBuilder) because:

1. It takes a generic parameter C as a stand-in for the client.
2. It holds a Box<dyn BlockImport>

There are a few possible paths here:

1. We try to make NetworkConfigBuilder serializable by moving the non-serializable parts into the build function, or
2. We make NetworkConfig serializable, which means the networking components need to take the unserializable things separately from the config,
3. We make a new slimmer version of NetworkConfig that is Serialize and Deserialize, and we have NetworkConfig hold one of these

Additional context

As part of this issue we would also need to deduplicate a few things. Currently we have a PeersConfig as part of the Config struct, but this is also present on NetworkConfig.

[leruaa]

I can take this on.