[settings] should be optional when loading from TOML

[jmg-duarte]

Issue Description

The following code fails because I don't have a [settings] field in the TOML.
zombienet (the original project) works without and so should the SDK.

 zombienet_sdk::NetworkConfig::load_from_toml(
            "local-testnet.toml",
        )
        .unwrap();

Steps to reproduce the issue

Steps to reproduce the issue

1. Don't put a [settings] field in a TOML file

Alternatively, load this example: https://github.com/paritytech/zombienet/blob/main/examples/0001-small-network.toml

Describe the results you received

Error because of the missing field

Describe the results you expected

A default [settings] should be used instead.

Serde allows for this.

Zombienet version

NA

Provider

Native

Provider version

NA

Upstream Latest Release

Yes

Additional environment details

NA

Additional information

I can help implement this

Screenshots

NA