[FR] - tx-generator allow restart

[ch1bo]

Internal

Describe the feature you'd like
I'd like the tx-generator to be able to restart / continue

Describe alternatives you've considered
Writing my own transaction submission / load testing client.

Additional context / screenshots
I just only started using this tool and thought the documentation is lacking: #6411

I'm using it in course of the demo setup for input-output-hk/ouroboros-leios#690