Revive load testing and run in daily CI

[letmaik]

https://github.com/microsoft/scitt-ccf-ledger/tree/main/test/load_test hasn't received much attention in a while and may be broken by now as it's not tested in CI.

The scripts should be checked and a new daily CI (like in CCF) added that runs the load test.

[letmaik]

I think a good option for this is to use locust as a library (https://docs.locust.io/en/stable/use-as-lib.html). This would allow us to write it as a regular pytest and re-use all the fixtures.
EDIT: From talking to CCF and the comment in the locust docs it is not advisable after all to use it as a library. It may make more sense to use CCF's perf testing library (C++) and run it such that it doesn't wait for commits, essentially trying to cause an overload.