feat: add parachains to a running network (#142)

This pr make possible to add a new parachain to a running network (see
`add_para.rs` example), for that we allow to create a new parachain
config (using the same builder but in a different context) like this:

```rs
let para_config = network.para_config_builder()
        .with_id(100)
        .with_default_command("polkadot-parachain")
        .with_collator(|c| {
            c.with_name("col-100-1")
        })
        .build()?;
```
And then deploy that parachain by calling `add_parachain` (the second
parameter of the fn allow to use a custom chain-spec for the
relaychain).

```rs
network.add_parachain(&para_config, None).await?;
```

// TODO:
- [x] docs
- [ ] test (integration)