benchmarks: Read SessionConfig from Environment

[ctsk]

Is your feature request related to a problem or challenge?

I think it would be useful if the benchmarks would consider environment variables for the datafusion configuration. This would let developers easily modify any datafusion setting and compare the performance, ideally even without having to recompile the benchmarks.

Thoughts?

Describe the solution you'd like

I want to run this:

export DATAFUSION_EXECUTION_TARGET_PARTITIONS=1
export DATAFUSION_EXECUTION_COALESCE_BATCHES=false
./bench.sh run tpch

... and have the tpch benchmark use the provided settings.

Describe alternatives you've considered

N/A

Additional context

I saw datafusion-cli already picks up these variables, but the benchmarks seem to not do that.