Improve serialization and configuration from file

[finnkauski]

Is your feature request related to a problem? Please describe. Configurating bridges from the existing constructors is a bit cumbersome. Ideally we'd have a more convenient storage format.

Describe the solution you'd like Implement some type of toml storage for the bridge and implement custom construction (most likely from serde::Deserialize. This is required if we decide to keep the Bridge.target field as it has to be created from IP and token at initialisation of Bridge.