Add optional `backends` parameter to benchmarking requests #3871

Job	Run time
Test codegen backend benchmarks	1m 20s
Test and deploy	4m 32s
Database Check	1m 30s
Compare page endpoint test	4m 24s
Test benchmarks (--include cargo-0.60.0,stm32f4-0.14.0,webrender-2022, Check,Debug,Doc)	6m 17s
Test on Windows	4m 31s
Test profiling	4m 27s
Test runtime benchmarks	4m 24s
Test benchmarks (--include cargo-0.60.0,stm32f4-0.14.0,webrender-2022, Opt)	5m 56s
Test benchmarks (--exclude cargo-0.60.0,stm32f4-0.14.0,webrender-2022, Check,Debug,Doc)	8m 46s
Test benchmarks (--exclude cargo-0.60.0,stm32f4-0.14.0,webrender-2022, Opt)	7m 49s
	53m 56s

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