[docs needed] Add option to spawn an reuse named spawned networks

[chevdor]

When writing specs or script, it is faster tio spawn a network once and run tests against this spawned network.

I propose the following changes:

Spawn a named network

Add support for a spawning a named network. For instance:

zombienet spawn --name net1 --provider [...] network.toml

Running tests on spawned networkd

Using the latest

zombienet test --spawned spec.feature

Using a previously started named network

zombienet test --spawned net1 spec.feature

[pepoviola]

This needs to be documented, but is already working. Changing the naming @chevdor. Thanks!!