initial lotus-compat testplan

[nonsense]

This PR is adding an initial testplan that automates the compatibility tests we want to run on Testground for Lotus, for example:

1. upgraded miner / old client
2. old client / upgraded miner

... and other similar compatibility tests, detailed under "Testground testing" at #6673

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You can run this testplan with: testground run composition -f ./_compositions/lotus-compat.toml assuming that your environment is connected to TaaS.

Logs from lotus daemon and lotus miner as well as the testplan are recorded in the testplan's outputs directory, so can be downloaded from TaaS after the test concludes.

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Known issues:

• the two versions of Lotus that we are running compatibility tests on are currently hardcoded in the Dockerfile - these need to be dynamic
• ideally we want a real-time access to logs - we could achieve this by exposing an http server on TaaS pointing to the relevant directory under /outputs - for now we have to wait for the test plan to conclude in order to view logs.

[magik6k]

Feel free to reopen if there is still need for this; today there seems to not be much need for testground tests