`fatxpool`: add heavy load tests based on zombienet (#7257)

# Description

Builds up towards addressing #5497 by creating some zombienet-sdk code
infra that can be used to spin regular networks, as described in the
fork aware transaction pool testing setup added here #7100. It will be
used for developing tests against such networks, and to also spawn them
on demand locally through tooling that will be developed in follow ups.

## Integration

Node/runtime developers can run tests based on the zombienet-sdk infra
that spins frequently used networks which can be used for analyzing
behavior of various node related components, like fork aware transaction
pool.

## Review Notes

- Uses ttxt API implemented here:
https://github.com/michalkucharczyk/tx-test-tool/pull/22/files
- currently, only two test scenarios are considered: 10k future & 10k
ready txs are sent to two separate networks - one parachain and one
relaychain, asserting at the end on the finalization of all 20k txs on
both networks.

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Signed-off-by: Iulian Barbu <iulian.barbu@parity.io>
Co-authored-by: Javier Viola <363911+pepoviola@users.noreply.github.com>
Co-authored-by: Michal Kucharczyk <1728078+michalkucharczyk@users.noreply.github.com>