[FEAT] Combine benchmark result JSON files

[SchoolGuy]

I as a user of pytest-benchmark
want to reuse benchmark results my CI gives me
so that I don't have to re-execute the benchmark locally.

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Currently, my GitHub CI is executing a part of the performance testsuite so I can parallelize the execution of it (it is quite long). To be able to compare this with a single local full run it would be cool if the standalone CLI could combine the different JSON files into a single "squashed" one to be able to have a one-by-one comparison with the local execution of it. Otherwise the comparison is quite tedious since you need to compare ten different test results with the local run.