[TM-320] Add a library for network testing

Description

• New library for network testing.
• Usage example for ManagedLedger.

It is only the first step, but it should already be sufficient to replace the scripts that we have in other projects. There are many improvements that we should make, see https://gitlab.com/morley-framework/morley/issues?label_name%5B%5D=nettest

It is currently based on !197 (merged)

Related issue(s)

https://issues.serokell.io/issue/TM-320

✅ Checklist for your Merge Request

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