Add read resource variable benchmarks.

Initial benchmarks:
entry {
  name: "MicroBenchmarks.benchmark_read_variable_op_2_by_2_CPU"
  iters: 30000
  wall_time: 22.9616721471
  extras {
    key: "examples_per_sec"
    value {
      double_value: 43550.8352176
    }
  }
}

entry {
  name: "MicroBenchmarks.benchmark_read_variable_op_with_tape_2_by_2_CPU"
  iters: 30000
  wall_time: 27.3616631826
  extras {
    key: "examples_per_sec"
    value {
      double_value: 36547.4859232
    }
  }
}

PiperOrigin-RevId: 184748548