Restore the serialization benchmark program

[Neverlord]

Bring back our serialization benchmark suite. I didn't just bring back the old version but also added a benchmark for the JSON serialization. The benchmark now gives us insights into the performance of our binary serialization of broker::variant and also the conversion to and from JSON.

Example run from my machine:

2024-05-12T16:29:04+02:00
Running ./build/broker/release/bin/broker-serialization-benchmark
Run on (12 X 24 MHz CPU s)
CPU Caches:
  L1 Data 64 KiB
  L1 Instruction 128 KiB
  L2 Unified 4096 KiB (x12)
Load Average: 12.03, 7.75, 5.86
----------------------------------------------------------------------
Benchmark                            Time             CPU   Iterations
----------------------------------------------------------------------
serialization/save_binary/0        123 ns          122 ns      5803397
serialization/save_binary/1        450 ns          446 ns      1570570
serialization/save_binary/2      15711 ns        15552 ns        45018
serialization/load_binary/0        129 ns          128 ns      5480353
serialization/load_binary/1        321 ns          318 ns      2202747
serialization/load_binary/2      37771 ns        37617 ns        18771
serialization/save_json/0          296 ns          293 ns      2370671
serialization/save_json/1         2832 ns         2812 ns       248191
serialization/save_json/2        86197 ns        85364 ns         8239
serialization/load_json/0          688 ns          684 ns      1015317
serialization/load_json/1         6476 ns         6421 ns       108800
serialization/load_json/2       186555 ns       184783 ns         3786

It's to be expected that JSON adds run-time overhead. But I think the slow-down is actually quite reasonable for Broker, better than I had expected.