bench: add COMMANDS suite with EXPLODE metrics

Shares the benchmark harness between both suites, extracts the plan/batch/apply helpers out of the EXPLODE command arm, and adds the command_benchmarks target.

Cargo.toml

@@ -151,3 +151,7 @@ strip = true
 [[bench]]
 name = "performance_benchmarks"
 harness = false
+
+[[bench]]
+name = "command_benchmarks"
+harness = false