[zombinet] introduce helper crate to support coordinated / relative block malus impls

[drahnr]

Problem:

For some test cases we require the possibility to coordinate between malicious actors. For this we need a "backchannel" to the test-runner needs to be implemented to provide key value based information as a bidirectional message stream that can be referenced.

Impl hints:

For compatibility and ease of use, values should be scale-encoded. Every value must have a key. Once a key is sent to the test-runner, it should also be sent to all other malicious entities, so it's a star network with the test-runner in the center.

Impl details:

• Provide a impl FusedStream interface to consume the updates.
• Provide a async send(key: &'static str, val: impl scale_codec::Encode) to send the value to the runner
• Assure no context is required, but only standalone fns, the context should be global/static.

Required for #4271

[drahnr]

Already implemented #4377