pipe generated tests into flood instead of creating intermediate json files

[sslivkoff]

Is your feature request related to a problem? Please describe.
the current solution creates 1 tmp json file for every request. this is inefficient for large tests

Describe the solution you'd like
piping the request data to vegeta directly without touching disk should be faster

Additional context
https://guozheng-ge.medium.com/replay-post-requests-with-json-payloads-using-vegeta-1dec356b7b51